The LM3875 chip from National Semiconductor is one of the most popular chip for Gainclone DIY, and given the proper power supply, can give you 50W. You can double that in a bridged or parallel configuration.The LM3886 can give you 60W and can also be doubled in bridged or parallel. Marchand has a kit based on this chip.
The chips are cheap. $7 to $8 each at retail. Do a search at digikey for exact pricing. It's the power transformer that's going to cost you. Chassis can be expensive too, depending on how nicely "packaged" you want things to be.
Gar.