One of my projects started something like OP's project did - a small gig, replacing a complex Excel with a simple software. The Excel was a way more complex tha

One of my projects started something like OP's project did - a small gig, replacing a complex Excel with a simple software. The Excel was a way more complex than I initially thought - business processes usually are, so take that into account if you offer a flat fee. Despite the flat fee for the initial project, I managed to do the replacement. After that the project grew and grew, and become fundamental part of their business - basically their ERP - and I ended up working on it for over 3 years. It was good money, but a bit boring in the end. I recommend that you set market rate as your base (target) rate. Ask around what the market rate is in your area, but it is likely more than $50/hour for short gigs like this. However offer them a significant discount, for example 50%, for the first X0 hours - and say that you offer it because this is your first project and it's win win for both parties, but you don't want to work with that rate forever. If the project grows in importance, and you get a lot more work from them, it is easier to negotiate new hourly price, when they don't expect that your standard rate is the discounted rate.