Do you have enough savings to cover you the rest of the time in school or will you have to get loans? What are your expenses now? I would say a good starting point is that moving in shouldn't cost you more than you're paying now (until you graduate and get a full time job) - so that means bf either needs to accept your budget and move into a place you can afford OR he needs to pick up anything above and beyond your budget. Unless you guys can agree on a place that would be cheaper for you - which would be great.

So say you currently are sharing an apartment and you pay $500 for rent and $100 that covers the utilities and you spend around $250 on groceries. If your bf is fine with finding a place that's $1000 total that similarly sized so utilities will be around the same and he spends similar on groceries - great! It's reasonable to split it 50/50 if it means your expenses don't go up.

Now if your bf won't agree to any place that's less than $2000 and is twice as big so utilities will be more and he insists on lobster and wagyu once a week - then he needs to pick up the extra above your budget. So you still pay $500 and he'll be paying $1500 for the apartment.

If you're currently living alone and you're looking only at one bedroom then it'd be reasonable to not want to pay as much as you are now to share the space with another human. Harder to come up with an easy number to select from that though. I'd say between 50-75% of what you are paying now as your max would be fair - depending on if that 75% gives you a better/bigger place or not.