This is always a difficult decision.
In Aug 2010 a supplier to my employer (whom I deal with regularly), approached me and wanted to purchase some "photos of Melbourne" for their newly built premises.
I have known them for the last 6 years so they aren't strangers but they are not my personal friends (per se).
I ended up selling them 6 images @ $150 each. I scaled the prints to suit the size they wanted (20" x 30" prints), saved as TIFF/JPEG and put the files on CD for them.
These were also images I had taken a while ago and were just sitting there. I spent a little bit of time re-PPing them before scaling but I didn't have to press a shutter button once
They were then free to take these to a printer of their own choosing to have them printed and framed and can display these indefinately at their premises.
Just prior to Xmas I managed to get out to their new premises and saw the prints hanging in their office space. Was VERY cool to see them printed so large and so prominently displayed for all to see.
Originally, I thought about going in at $50 per image (after they had selected 6 of the 10 or so I had sent them to review), but thought that people/companies pay $100's if not $1000's for some artwork to hang on their walls so why should mine be any different. They are absolutely wrapped with the images and the money certainly came in handy for me.
So, I'd carefully consider what you feel the images are worth, what final products will be handed over (digital files for printing or actual framed prints where YOU do all the leg-work to have them printed & framed) and I guess if they are friends do you then offer a discount.
In my case, I didn't really have the time (at that stage) to organise printing/framing etc. and they were more than happy to arrange their own so it worked out perfectly. In your case, your colleague may not want to do that so you will have to then find a good printer/framer, get the files to them, arrange delivery or pick-up, then deliver prints to your colleague (all of whiuch I'd want to factor into the price - along with the actual artwork itself).
Hope this has given you something else to consider...
Dave