Here are the main differences:
- Magento uses mostly blocks for its modules and components so you have to understand how they work and how they fit with your HTML and the overall code. It’s not plug and play and it takes time to figure out. Even after several projects.
- You have to take into account in your evaluation and work all the pages beyond the home page and product pages, like account log in, checkout, shopping cart, wishlist and so forth….
- Don’t forget the “communication” pages like transactional emails, newsletter templates, etc…
Overall, creating Magento templates are easily twice or triple the time it takes for a normal “PSD to HTML” job so be careful before you quote anything if you don’t want to lose your shirt. There a lot more pages and templates to play with than the home and product pages.