I can only think of a few people that designed their own website that got traffic from the search engines that converted into leads/jobs. I've seen plenty of good looking sites that nobody can find when they search MSN/Google/Yahoo. I've also seen people get lucky with SEO and not convert visitors because of a bad site.
While there is not any connection between the amount of money you spend and the amount of leads that you get.....there is a little proof that a professional looking website with good/great SEO will bring in more leads.
Let's say contractor #1 spends $1000 on his/her website and SEO and gets 50 leads in the first year from the search engines alone, his/her lead cost is $20 each (excluding hosting.......)
Let's say contractor #2 spends $20 on a self design package and donates 50 hours of his/her time in the design. At the end of the first year contractor #2 gets 1 lead from the search engines for a lead cost of $20 (excluding hosting, donated time.....)
Each contractor has the same lead cost but one contractor has 49 other leads to play with to recoup his $980 loss. If you are going to take on the project on your own, start a thread "who designed their own website that gets them a bunch of organic traffic....that converts into leads/jobs". Matt from Contractortalk will one of the few that will answer.