A better plan (if you want to minimise the amount you have pay designers - although, paying a designer is the best option) get them to go a base design/ style guide (and maybe a screen or two) then learn the grid rules and finish it off.
Grid and colour rules are easy. Design isn't. Although the same can be said for development.
This brings up other issues that are generally the main reasons people build web apps in the first place:
1) Updating code is a pain. You can't just push a patch, you have to rely on others to download the patches and install them
2) You now have to support multiple versions. Because of above, you'll be getting bug reports for old versions
3) Different hosting environments. Not only wil you end up with support for environments you've never heard of, but you'll have to contend with bug reports that are actually to do with misconfigured servers.
4) Unless you are targeting enterprise (in which case there IS a business case for self hosting) the number of organizations with the know how and resources is probably far lower than the rest of your user base - and they will probably take up most of your support time.
5) You lose the subscription model. Certainly some apps can charge people on going fees for support etc, but subscriptions are such an easy way to make money in software.
Your point is valid, but it kind of goes against why people chose to build web apps!
Although, I wonder if it is because I'm a professional computer programmer? It seems these desktops are getting simpler in-lieu of settings and customisations? Fine for casual users but for us power users it becomes more and more frustrating.
I think the "post-pc" era is going to be difficult for developers and other computer professionals.
I'll let them know I take credit cards.