Affordable doesn't have to mean sacrificial. You should be able to try out a mixing or mastering engineer to find out if you even work well together before you attempt to work with them. Also I believe that those selling services should not tack that 'free' cost on to the back end. I'll mix your first song for free, and then we can go from there.
I have a lengthy set of skills surrounding songwriting and mixing as well as song composition. I ran The Noisemaker Podcast for several years as well as released a book on songwriting called The Fat-Free Guide to Songwriting.
Tell me about your project and how I can help, through the 'Contact' button above.