First off, read tptacek's excellent answer over and over until you can recite it from memory. I have been doing freelance consulting for years and I know I'm going to steal a number of his ideas.

Second, here are some additional thoughts, based on my experience doing similarly-sized gigs for similar clients.

1) To start with, just to encourage you, this is a very doable project that can be very lucrative to you (>$10k), helpful to the client, and can open the door to a lot of fun side gigs. This is the best part of consulting.

2) When looking at your first side job, the temptation will be to come up with a price based on your day job's equivalent hourly rate. Do not do this. This is easily $100+/hr work. For a real estate firm, your app may be generating millions in revenue/cost savings over a few years, and although it's a simple little project to you, building a webapp is deep magick to a business user. Try to pitch and charge based on value.

You should know your costs/equivalent hourly rate, but only as a bare minimum so you are never working at a loss.

3) Based on the fact that this project is mostly rebuilding an existing Excel spreadsheet (Pro Tip: you can get a lot of business from simply "rebuilding Excel worksheets!"), it seems like you could spec it out enough where you could offer a fixed price. You can use tptacek's approach of quoting X days at $YYYY per day, or a total fixed price.

You always have the option of going hourly though. Some clients will prefer this, but it can limit your total compensation. You can charge $X0,000 - $X00,000 for a given app (remember: deep magick) but you can't charge $X000 per hour. Do quote high though. My favorite advice is "pick the most ridiculous semi-realistic number you can think of, then double it." You can always offer a discount for Friends & Family, New Customer, Summer Sale, whatever.

4) Be vague about delivery times if you cannot take 1-2 weeks off work to complete the project. But plan to have regular deliverables/signs of life to keep them on board. Think about mockups/wireframes you can deliver.

5) Ask for at least some of the payment up-front. This will ensure they are serious, and motivate you to deliver once you receive money. :)

Home these ramblings are helpful. Welcome to the world of Real Consulting!