Price-wise, you might be well off to go to Joann Fabric if there's one nearby. They always have 40% off coupons online, and if the foam isn't on sale already you can use the coupon on that. I did a reupholsery job a while back and used a combo of straight razor and electric knife. Worked pretty easily and the edges came out relatively clean (I could have done better, but they weren't visible on the end product so I didn't try).
Good luck!