I don't master songs for free. I don't do shootouts. 99% of my biz is word of mouth, most clients pay full in advance, although I only require a 50% deposit of the projected cost before I start. Ads in Mix mag don't do squat for generating work, great for getting resume's. You want to find a good intern, put a mastering services ad in one of the trade mags, but don't expect to get much work from it. My website gets lots of hits, but maybe 1 out of 500 actually follow through with a project. Yep, it's all word of mouth. Your best business card is the last cd that you mastered.
I wonder how many PC recording jockey's, send out 10 different songs to 10 me's for their demonstration, or tell them it's a shootout and try to put them all on the final cd. It's a funny thought but certainly a few have attempted this. It's always the people that want something for free that aren't worth messing with in the first place. Producers that know their stuff, band alpha members, self producing bands and even home recordists that put out decent material, never ask for a freebie. It's a red flag, if they don't want to pay for the best mastering than it's very likely that their material is less than good anyway. The last place you want to cut corners on a cd production is the mastering. You get what you pay for in the real world.