The best cheap audio plugins for recording, mixing, and producing old school retro music:
This odd bird compressor is part La2a, a pinch 1176, and 1/3 "tube driver" saturation. Some downloaded it free as a tribute to the founder of Nomad Factory. Others paid $5 through JRR Shop. Now it's not even listed on the Nomad page, but is still available elsewhere. Your best bet is to Google it and see who has the best current deal. If you can find it on the cheap, snag it up!
Try it after an 1176 to smooth things out. It brings some glue and grit to tracks, and feathering-in the "tube driver" is a lot of fun. Takes clean and sterile sources and makes them dark and gooey. Dig it.
Perhaps a bit more refined than the similar Bus Driver above, Klanghelm's most talked about darling is cheap. At around $25, you can't beat it with a stick! It includes 3 different compressors - old school vari-mu designs hearkening back to analog compressors of the 50s and 60s. Pick MK1, MK2, or MK3, and mess with the "Timbre" and "Drive." Super fun and a lot less stress on the pocket compared to some comparable plugins like Universal Audio's Fairchild.
WAVES J37 is a familiar face in the audio plugin world. It seems everyone has a tape machine simulation. If you write and record a lot of "old school" or "retro" music, we would recommend this fella as a contender. It is a bit more vintage sounding to our ears compared to UAD stuff. Anyone who knows Waves, know their sales can swing prices in all directions. If you check once in a while, this puppy has come down below $50. If it works for you, then it will be a plugin that gets a lot of action.
Plug and Mix plugins have that basic turn-a-knob appeal. This is a nifty maximizer to bring your master level up to broadcast mix level. Easy to use with a pleasing GUI, and it gets the job done. They also have a cool series of "Retro" plugins.