The very first thing to say, when asking for legal advice, and the first thing to think about is which legal jurisdiction you are working in. If you are based in the United States the various contract laws of many of the states are pretty similar but they are by no means identical. The laws of other countries - for example in Europe or the Far East - diverge a great deal more.
This is just as important as saying what programming language you are using when seeking advice on a site like stackoverlfow.
Almost all lawyers in this field (and I am one) do exactly what Joel suggests: we reuse each other's work. In most cases we start off with books of precedents and then rewrite them to suit our needs, picking up ideas from other contracts we see out there and taking input from clients as well. A good firm with lots of experience will have plenty of its own "in house" material, but that will often include a considerable amount of commonly used material (what we call "boilerplate") that is out there.
Obviously editing other people's contracts might be a copyright violation depending on your jurisdiction and how original they were in the first place, but certainly in my jurisdiction (England) people aren't often sued for such violations. If you do it with permission, all the better of course.
If you go to see a lawyer they will appreciate your having done some preparatory work. In particular you will need to think carefully about things like:
- how payment will work (not just how much but when and how)
- how the contract will end (do you give notice or can you cancel it if they don't pay etc)
- what exactly you are promising to do - in the case of SaaS you will probably be guaranteeing a certain level of service availability, but you'll be taking the site down some times for emergency maintenance (if you are anything like any of the sites I know) and you'll want to make it clear in the contract what you can and cannot do like that
- how will terms change - eg prices increase - over time?
- is there any element of confidentiality (always a tricky point)
.... and so on.
Also rules differ for commercial and consumer contracts in many places (in Europe for instance) and that might be a factor.
If you have thought through all this, and perhaps produced a rough draft that makes this all clear, and present it to a competent lawyer then you should save lots of time and money.
A good lawyer ought to be upfront about their experience. Ask them if they drafted any SaaS or similar contracts before. Contract drafting is mostly standard stuff, but there are peculiarities of websites that make a difference.
Rates will vary depending on where you are (see above).
If you are in England or Wales (unlikely I know) then you are welcome to have a copy of one of the contracts I use as the (or one of) the templates Joel suggests using. If anywhere else, I can't help you.