Just wondering what the cheapest NES proto is.

Do you mean in regards to previous sales, or the cheapest to currently obtain? If you mean previously, do you mean private sales, open auction, or buy it now?

The first question is pretty much impossible to answer with any accuracy, but common titles of unpopular and non-current series tend to go lower than most others. With enough digging and patience, you could likely get an NES proto for around $50, especially if the major proto collectors already own copies of the game.

I meant what's the cheapest worth.

I'd like to own at least one is all. But I guess that's probably the case with everybody else.

Worth is a tough thing to quantify, but if you 100% don't care at all what the title is, I stick to my $50ish prediction for the lowest you'd likely be able to snag one. There are plenty of factors that would influence this (like who already owns/doesn't own a copy of the particular proto) as I said earlier, but with patience you could likely do it.

Good luck on the search, hope you find something cheap that you like!

Generally, unlicensed protos usually sell for the least amount (from what I've seen). In contrast, anything unreleased, or even better, unreleased and undumped, will be worth quite a bit.  Also, first-party protos (Super Mario Bros., etc.) generally fetch a pretty crazy price.

I can understand first party protos. But wouldnt unlicensed ones be the most expensive?

Nope, and with good reason.

Many unlicensed games contain EPROMs to begin with, so telling a prototype from a production pcb can be a chore.  Proving the prototypes legitimacy can be even more difficult. 

Unlicensed titles tend to command high prices due to their relative rarity, but in the world of prototype collecting all games are rare.  Think of it this way, if there's we're hundreds of thousands of copies of Action 52 out there, it wouldn't be worth a fraction of the current going rate.  Now imagine if the production of SMB3 was as limited as The Caltron 6-1.  The value would be exceptional.