Some additional tidbits that bear mentioning.

• Who is the customer? A small business that intends to use this application heavily in critical business processes, a quasi-governmental org that just needs something functional in order to qualify for grant money?
• Who is on the hotseat for technical support, bugfixes, maintenance and upgrades? What are the terms of the contract, how thorough is the design specification?
• What would this customer consider the next best alternative if they didn't have the option of whatever it is you are giving them?

Unless you are building something exceptionally unique, bear in mind that there are zillions of services and software out there that allow deployment of run-of-the mill web database front-ends out there ... many requiring little or no technical proficiency.

There are also programmers and design firms out there with many decades of experience that will be more than happy to charge thousands for very specialized and very high quality work.

There are also websites that allow people to buy 'works for hire' internationally ... and in many countries, (qualified) people are willing to program for what the USA considers "minimum wage" (some even less).

The main point is, asking how much a web-app is worth is somewhat like asking how much is a meal at a restaurant worth. Depends. Are you talking McDonalds? The Beverly Hills Chateau du Sureau? Even if you design the app flawlessly with industry best practices, that doesn't make much difference if the person paying the bills is not sophisticated enough to appreciate that fact, especially if you give them filet mingon, and all they wanted was a big mac and fries.