Never go with the first estimate. This may seem like a no-brainer, but when you're eager to get started on something like replacing your roof before the next big storm, it's tempting to just hire the first contractor you talk to. I've heard that three estimates is a good rule of thumb, but in my experience it takes more like five quotes to really give you an idea of the price range out there.
When we needed a new roof, I was very close to hiring the first contractor that gave us a quote, but my husband convinced me to make a few more appointments with roofers. That first quote ended up being almost double any of the others! Not only did talking to several contractors keep us from overpaying, but we were open with them about getting several quotes, and some offered to price match.
Shopping around doesn't just go for hiring workers. You also want to get mortgage quotes from more than one bank. Shaving even part of a percentage point off of your loan can cut down on your new mortgage payment. There are also different renovation loans that you can look into, like the 203(k), a home equity line of credit, or just plain refinancing your existing mortgage. Shopping for a loan is no fun, but finding the right one can save you a lot of money!