Also, don't be a 'freelancer.' A 'freelancer' is a mindless drone who is told to do something and does it. Be a consultant. A consultant is an expert who provides value to the business, either in terms of time / money saved, or increased revenue. How you market yourself is important.
Here's a gem from tptacek: http://news.ycombinator.com/item?id=4245960
Also, go to hnsearch.com and do a search for 'patio11'. Read everything he has written - it covers marketing yourself and charging appropriately. Do it now. That proposal you're working on can wait :)
Good luck, and welcome to the crew!
Edit: Freelancers vs Contractors vs Consultants: http://news.ycombinator.com/item?id=3420396
I have read everything patio11 has written about consulting , twice. As I've said on HN before his writing has changed my life in a few dramatic and interesting ways. Specifically this guy: http://www.kalzumeus.com/2011/07/08/business-psychology/
I somehow missed that link you posted by tptacek but it truly is a gem. I'm at about stage 2 of that now and it's exactly what I needed to hear.