The real point here is that this isn't simply hashed together code though. You're intended target audience is students, which if they're moving from FPSCreator would suggest they're not exactly programming-centric.
As such this adds a whole new level to what is written.
For example, a while back Kenjar asked me to make a Space Invaders clone with the intent that the source code would be seen by people learning to program for the first time.
Making a clone that worked close to identical to the original took around 3 hours (but only because I spent most of that pissing about making a BBC BASIC style SOUND function), yet then I had to restructure that code from my style into something that was easier to read. Then put in more understandable comments so atleast Kenjar would understand what was going on at a glance, etc.
In the end it all took just over a week to complete (although it was only about 1hr a day I could really dedicate to it, including testing throughout the code)
Point is though, it takes more to write code for educational purposes than you'd think. I mean I also had some design docets and such that really weren't needed but felt that would help to explain in a more visual way what was happening and changed.
Still you have to think about what each part does, what exactly it's really going to teaching, etc.
So it might take longer than you expect from that respect. Although a plus side a training tool doesn't need to exactly be top-notch quality in terms of playability and such (although most I know who have pride in their work will tend to do such sub-conciously).
Think about what work you're putting in to begin with.
As people have said, go by minimum wage for your base estimate with your estimate on time-scale.
I'd say 2 weeks (10 working days, 8hrs a day) should be a good estimate for something like this. Remember that things can crop up and take longer than you might expect, this also gives you enough time to look over everything. Make sure it does what it does, crush bugs that appear, work around issues that might appear, etc...
Dunno about your age group, but the minimum wage in the UK:
18-20: £4.90
21-24: £5.60
25+: £6.20
I'd suggest going by the top-end simply cause well often it's good to over estimate as it gives you that little extra
Also as you're not providing IP for this, and you are offering it as what effectively is a non-commercial license agreement then add on a "bonus" i.e. guarenteed minimum payment no matter how long it takes.. but also offer as the "NCLA" and also offer a "Royalty Free Agreement" price (which I'd suggest should be 5-10x what your NCLA is) this way while you're getting extra from the deal, the customer feels they're getting a deal going for the cheaper option.
Lastly you should probably note that professional programmers generally earn in the region of £10-25/hr (depending on the company) where-as most freelance/database programmers will earn closer to £25-50/hr.
This again is something you can work by, and not that because they are an education institute you're providing your services for half the price you normally would. Again they'll feel like they are getting a better deal.
If the guy/girl in charge of the course and contracting you has any sort of industry experience, they will know roughly the going rate; so will probably expect something fairly high.
You generally charge more for such services being freelance because work is not guarenteed. If it was guarenteed then you charge less. You could also encourage this for further business by offering your services at a discount next time (say minus the bonus)
Also I would suggest contacting TGC as they deal with educational licenses directly (I believe Rick does atleast) and may be able to provide you with information on other institues you could further retail the source code to.
Remember to make sure you have a contracted agreement prior to starting the work, and also make sure you provide a license with your source code that states what they're legally allowed to do with the source and that anything made based on it is under the same license. (Creative Commons License is actually a very good one for source like this, as is LGPL)
