Fair enough :)
Getting four of the Exit games will fit your budget, should give you about that much entertainment, and they are IMO the best of the bunch when it comes to the exit room games. But there’s a couple of things you should be aware of:
I don’t think the plan of having her play with you, and then later play again with her friends at college would work. It’s not very fun for either side. The player who has played the scenario before realistically can’t really do or say anything without giving away the twists.
And in addition to that, the Exit series is designed for you to destroy the components while playing. It’s possible to work around with various kludges, but honestly it’s not a great experience unless you have a photocopier.
If you’re looking for something more in the “detective mystery” rather than “exit room mystery” style, I have mini reviews of everything I’ve played in that genre in the detective games thread.
Basically the Consulting Detective games are still the gold standard there, and some of the box sets should be readily available. Do not get the Jack the Ripper one, it’s pretty disgusting and not fun to play at all. Playing every scenario in a box should be a good 25-30 hours, and if one player has played before it’s not the end of the world since they can just go and chase one of the other plot threads without spoiling the game for the others.
Detective: A Modern Crime Board Game is probably the most successful of the new games. It’s got some problems, but is a pretty compelling experience despite that. It’ll be about 15-20 hours, and all the scenarios basically have to be played with the same players. A new player will not be able to effectively enter the game in the middle of the campaign. So it’s another one where I don’t think it’d be something she can take to college after playing with you.
I’ll anti-recommend Chronicles of Crime.