FreedomPop is an MVNO cell phone service that offers a free cell phone plan with 200 minutes, 500 texts, and 500 MB worth of data. (And you can set it up so it cuts off once you reach the limit, rather than getting hit with overage fees.) So if you buy a mobile hotspot from them, like this one for $30, you have a device that entitles you to 500 MB per month of free data for life (or more realistically until they go out of business).
But my question is, do any other cell phone services sell mobile hotspots that entitle you to free monthly data forever? The only other MVNO cell phone service I know of that has a free cell phone plan is CellNuvo, but they don't sell mobile hotspots, only cell phones, and their cheapest cell phone is $40 with a $29 activation fee. (There was another MVNO with a great free cell phone plan called RingPlus, but they recently went out of business.)
EDIT: I don't really have a budget, but of course the cheaper the better, and the more free data per month the better. As I said above, FreedomPop offers a device that costs $30 and gives 500 MB per month, so preferably I want something that's better than that in or of both of those dimensions. Also, I live in New Jersey, so that's going to be the main place I use it in.