Free-Voucher.com: good name but i dion't like the hyphen. mid $xx to low $xxx
VoucherInfo.com: this is one of these names that has no direct value by itself as a name BUT it is great for develoment so the real value is totally in your own hands. It's very memorizable so if you develop it well and get visitors they won't forget it and come back on a regular bases.
CouponFact.com: same as VoucherInfo with one bad thing, i can imagine alot of people typing couponfactS.com if not visited from the favourites in the browser