There is always the router planing jig, or for roughing, an electric hand plane if you want to use power tools, or the solution since Roman times, a simple hand plane. If you are not skilled with hand planes, this might not be the best option for this project, but maybe the future?
Check craigslist, you can find decent planers for well under $400 pretty often. I picked up a Jet 6" long bed jointer and a Rigid 13" planer for less than $400. As Wolf & Rabbit asked…what needs planing?