Congratulations on your first job @
tarynmahon
When I got my first consulting project, I was not sure how much to charge either. So I reached out to my mentor (Jon Peltier) who gave me some valuable advice. Since I had a full time job (and was doing consulting project mainly to enhance my skills), I charged $30 (US) per hour. I billed the client for about 8 hours. It was a big chunk of money for me (remember, I live in India, where $30 can pay for rent if you live modestly.) This was back in late 2008.
Later I went to $50 per hour (in 2009), $75 per hour in 2010 and $100 per hour in 2011. Finally, when I stopped doing any consulting work in late 2012, I was charging $150.
My suggestion is to go with hourly rate if you are not sure how long the work takes. Make sure you communicate with the client often and warn them about potential extra billing early on. This will help you gain their trust and award you more projects.
Also, give them an estimated number of hours / cost as soon as possible. This way even though you are working per hour, they will believe that they are on fixed price.
To give you a rough ball park, most Excel consultants (MVPs, seasoned pros) charge between $150-$300 per hour for development work.
Once you gain some experience and trust, I suggest going for fixed price projects. This will let you build and deliver models based on
how much they think the model is worth rather than
how much they think your time is worth. And believe me, a model that takes 8 hours to build can save any meaningful client at least 80 hours of time.