You might want to do some investigation on small linear amps that can be built for under <$100. uBitx.net has some references and there are several choices. There is one designed by a ham that won some price from the ARRL a while back that gets good reviews. (Wish I could remember his call letters - any help here?) Several ham kit suppliers, including the folks that make the cases for the ubitx sell a two board set (amp + filter). You can also get some on ebay for under $20, but you have to put them together. As designed, they are not that great. However, there are several blogs/websites that will help you fix them so they are respectable and on 12v give you between 40-60w (depending on band).
Because I like a challenge, I bought the inexpensive one on ebay. Search for
DIY Kits 70w SSB Linear HF Power Amplifier for Yaesu Ft-817 Kx3