This fundamentally boils down to a motherboard question - what is the cheapest motherboard that can inexpensively support at least 512Gb RAM. Finding the absolute cheapest would be difficult, but I can suggest one that will not disappoint in making your total system cost a lot less than $13,000 (disgusting, innit?). Really this is going to come down to how great your ebay skills are, because the cost is going to utterly depend on the RAM modules you can acquire -and while you can reliably buy high-capacity DDR4 in stores for relatively cheap, you can sometimes find high-capacity DDR3 for cheap enough to justify that route. I will provide solutions for both paths.
First, the DDR4 build - it depends on the Asus Z10PE-D16. Please note that the rest of the build surrounding this motherboard is only to give you an idea of total system cost and likely configuration, and may not reflect a "good" design for your particular use case (Since I have no idea what you're doing, I'm just building it as I would for a sort of general use case in the vicinity of what you appear to be trying to accomplish).
PCPartPicker part list: http://pcpartpicker.com/list/Vw94d6
Price breakdown by merchant: http://pcpartpicker.com/list/Vw94d6/by_merchant/
CPU: Intel Xeon E5-2603 V3 1.6GHz 6-Core OEM/Tray Processor ($179.61 @ Amazon)
CPU: Intel Xeon E5-2603 V3 1.6GHz 6-Core OEM/Tray Processor ($179.61 @ Amazon)
CPU Cooler: Intel BXRTS2011AC CPU Cooler ($18.99 @ SuperBiiz)
CPU Cooler: Intel BXRTS2011AC CPU Cooler ($18.99 @ SuperBiiz)
Motherboard: Asus Z10PE-D16 SSI EEB Dual-CPU LGA2011-3 Narrow Motherboard ($389.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Memory: Kingston ValueRAM 32GB (1 x 32GB) Registered DDR4-2133 Memory ($162.99 @ SuperBiiz)
Storage: MyDigitalSSD Super Cache 2 64GB M.2-2242 Solid State Drive ($36.99 @ Newegg)
Storage: Hitachi Deskstar 7K2000 2TB 3.5" 7200RPM Internal Hard Drive ($50.00 @ Amazon)
Storage: Hitachi Deskstar 7K2000 2TB 3.5" 7200RPM Internal Hard Drive ($50.00 @ Amazon)
Storage: Hitachi Deskstar 7K2000 2TB 3.5" 7200RPM Internal Hard Drive ($50.00 @ Amazon)
Video Card: PowerColor Radeon HD 5450 1GB Video Card ($17.98 @ Newegg)
Case: Phanteks Enthoo Pro ATX Full Tower Case ($74.99 @ Newegg)
Power Supply: Antec HCG M 850W 80+ Bronze Certified Semi-Modular ATX Power Supply ($90.98 @ Directron)
Case Fan: NZXT FN-200RB 166.2 CFM 200mm Fan ($18.89 @ OutletPC)
Case Fan: Scythe SY1225DB12SH 110.3 CFM 120mm Fan ($14.89 @ OutletPC)
Case Fan: Scythe SY1225DB12SH 110.3 CFM 120mm Fan ($14.89 @ OutletPC)
Total: $3814.64
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-08-21 03:54 EDT-0400
For the DDR3 route, I recommend the Supermicro H8DG6-F-O (sorry I can't put it in block quote because I needed links in there):
PCPartPicker part list: http://pcpartpicker.com/list/ytc9NN
Price breakdown by merchant: http://pcpartpicker.com/list/ytc9NN/by_merchant/
- Motherboard: Supermicro H8DG6-F EATX Dual-CPU G34 Motherboard ($537.99 @ SuperBiiz)
- Storage: Sandisk ReadyCache 32GB 2.5" Solid State Drive ($58.17 @ Amazon)
- Storage: Hitachi Deskstar 7K2000 2TB 3.5" 7200RPM Internal Hard Drive ($50.00 @ Amazon)
- Storage: Hitachi Deskstar 7K2000 2TB 3.5" 7200RPM Internal Hard Drive ($50.00 @ Amazon)
- Storage: Hitachi Deskstar 7K2000 2TB 3.5" 7200RPM Internal Hard Drive ($50.00 @ Amazon)
- Video Card: PowerColor Radeon HD 5450 1GB Video Card ($17.98 @ Newegg)
- Case: Thermaltake Core V51 ATX Mid Tower Case ($59.99 @ Newegg)
- Power Supply: Antec HCG M 850W 80+ Bronze Certified Semi-Modular ATX Power Supply ($90.98 @ Directron)
- Case Fan: NZXT FN-200RB 166.2 CFM 200mm Fan ($18.89 @ OutletPC)
- Case Fan: Fractal Design HF14-BK 118.2 CFM 140mm Fan ($17.99 @ Newegg)
- Other: AMD Opteron 6136 ($5.49)
- Other: AMD Opteron 6136 ($5.49)
- Other: Dynatron A13 60mm 2 Ball CPU Cooler ($29.99)
- Other: Dynatron A13 60mm 2 Ball CPU Cooler ($29.99)
- Other: 16x AXIOM LV409672A8D3L13811 32GB RDIMM LDDR3 ($2279.20)
Total: $3302.15
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2016-08-21 04:11 EDT-0400
Again, if what you say is really true and you don't need to worry about CPU performance, then this latter option might actually be a really cool buy if you can hunt down the right parts and prices. Currently there doesn't seem to be a whole lot of 1x32gb DDR3 RAM going around at acceptable prices, but if you can source that then you've pretty much got it made.
I might also note that with 512gb of RAM in play, you're almost certainly going to want ECC RAM - both of my options are ECC - and you'll also want to consider the sanity of whatever it is you're doing. Chances are there's some clever way to cache or distribute your data so that it doesn't have to ALL live in memory simultaneously on a single machine. If you can figure that problem out then you can save a bundle.