Cheapest wheels would be used ones. Either stock V6 or stock 16X7 wheels work good, or a used set of drag wheels like welds and such. Just be forwarned that most of the weld wheels require longer wheels studs to work on our cars which adds to the expense.
As far as back spacing, a good rule of thumb is that you don't want the total of the wheel width minus the back spacing to exceed 4" MAX. I personally wouldn't let it exceed 3.75" Also, you can not fit anything much wider than 6.9" of back spacing under the car.
For example, my 10.5" wheels have a 6.75" BS which give me 3.75" from the wheel hub face to the furthest out portion of the wheel. Most of the 10.5" cobra R wheels and such have a 6.95" BS which yields 3.55" from the hib face to the outermost part of the wheel. If you are putting a 10.5" wide wheel on the car you will want to stay between 6.75 and 6.95 MAX to avoid clearance issues. Just remember, the 6.75 moves more of the wheel to the outside of the car (to the fender) and the 6.95 tucks it more underneath the wheel well. If you get into running 28" high tires, you will want to find out from others EXACTLY what BS is the best fit as with the 6.75 it may or may not hit the fender, and with the 6.95" it may hit the inner fender, quad shocks, etc. This info is all for a 10.5" wheel but it can be applied to any size wheel. For example on a 7" wheel you can have as little as 3.25" of BS and it will still fit in the wheel well but it'll push the tire out to the fender. Just remember that you want the value from the axle hub to the outtermost portion of the wheel to not exceed 4" MAX and the BS can never exceed 7" MAX and you should be Ok with the fitment depending on the size of the tire. When I say MAX I mean MAX... I wouldn't get too close to the numbers. To be on the safe side I personally wouldn't do much more than 3.75" max from the hub to the outer portion of the wheel and 6.9" of back spacing.
