I imagine a Harbor Freight 8x12 would fall in your category of price and size, assuming you can come up with a 20% off coupon from HF. Regular price is $550. You could get a 7x12, but in all honesty, the 8x12 is probably a MUCH more capable machine for relatively little extra cost. I opted for the 7x10 for it's small footprint and ease of moving by myself.
My guess is the 8x12 is going to be your best value in the $500 price range.
Hopefully Teyber12 will chime in here, as I think he has this lathe and is pretty happy with it.