try calling some local upholstery shops around you, tell them you want to increase the height and ride feel, you want to add foam and have them re cover it for you. they should give you a good price because its easier to just recover it then try to stuff in more material and use the same cover it came with.
any of the new seat "kits" from what ive seen are 4-700$ not including shipping, i had a buddy reupholster a seat for like 250$, you shouldnt be much more.