according to the price guide on this site the cheapest one is KOF95. FF3 and SS3 are also cheap. but it depends which ebay seller u buy from, and what shipping u pay, and what condition the cart is in- often the cheap snapcases have defects like small dents in the plastic insert/insert cover.
price guide: (complete, near mint)
KOF95: 60-85
FF3: 60-95
SS3: 65-100