Hi Ali
I haven't had a lot of experience but you'll realize not too far into your flip that you really can't estimate on a square foot basis and be successful (or prepared). There are definitely ranges that prices fall into but you are still going to have to go through the tough work of itemizing everything and estimating it.
For that reason, I'm going to recommend the things I think will give you the info you need to make you a better flipper.
First, I'd recommend "The book on estimating costs" as mentioned earlier. If you order "The book on flipping houses" from Bigger Pockets thru the tab above (rather than Amazon etc), you will get the estimating book thrown in for free. Both books are like buying confidence.
I recently purchased the spreadsheets, etc. from HouseFlippingSpreadsheet.com and am very pleased with it. It has way more than I thought it would - all kinds of stuff.
And finally, another awesome tool i've recently been using is Homewyse.com Between those things you'll feel really in control of your flip and give it a much better chance of working out for you.
The only way you can really get out of doing the tough work is by hiring a GC and letting them do all the estimating for you. If I were you, I'd wait until you have worked quite closely with a contractor a couple of times before doing that.
Best of luck Ali!!