I am in the process launch one of my latest websites. I purchased off the shelf website software that is being cut and modified by web developers. This is now my sixth running website business. I have to tell you that if you are not familiar with building websites and don't know the in's and outs of what you want paying a lump summ to get your website up and running will often leave you with additional development costs as you thought process on how you want your website to run evolves.
Also paying by the hour is also somewhat loaded. If you are not 100% crystal clear on what you want done, your website costs will spiral out of control.
As a rule of thumb, that what you had budgeted for a realistic, well working website and double it to get what you ideally want. I know this sounds kina dumb but it's really not.
Here are some pointers on how to get what you want:
1. Spend as much time as you need writing a business plan
2. Spend weeks researching your competition and how thier website functions, log all the good points and throw out the bad.
3. Incorporate this new information into your business plan.
4. Take adequate time to whiteboard how your website will work and record how your website is navigated.
5. Take a day to read about how ecommerce works, open a google ad words account, and learn something about SEO
6. Take adequate time to document all the specifications on the website (Most important)
7. Pick a look and feel from a website like templatemonster.com
8. See what off the shelf software is available that you can buy and modify. The first site I built from scratch cost me around $135K to build
9. Interview at least 5 web dev firms.
10. Sign contracts based on your written specifications.
11. Split the cost of the website over 3 payments with the last payment being made 10 days after you have debugged the site.
Hope this helps