You mentioned that she invited you. Next time she invites you, you could respond like this:
Yes, I would love to see you and spend some time with you. My money is tight right now, so I can't afford to travel to [her location] and pay for dinner for both of us. Can we split the cost of dinner, or is there something we can do that doesn't cost much? What I really want is to spend time with you.
When she comes to your town, you have more control over where you go and what you do. Don't go to an expensive restaurant without establishing who will be paying. You can construct a great visit with less expensive options. If she says "I don't want to eat at X, why can't we go to somewhere like Y instead?" you can easily reply
On a student budget I can't afford to eat there.
If she has a lot of money she may reply that she can pay, or she may suggest another place or agree to the place you suggested. I suppose it's possible at some point that she will think to herself "if Gasper isn't going to take me to expensive restaurants and pay for everything, I don't want to date him" but I only ever met one woman like that in my 50+ years on the planet, and she wasn't looking for long term relationships anyway, just fancy meals in expensive restaurants. My point is if you are broken up with over not buying a lot of expensive restaurant meals, it's not much of a loss.
Now, disclaimer. You say 15 euros is a lot for you. If you can't afford any of the traditional date activities, not even some street food or 5 euros admission to something, then it's going to be really hard to construct activities the two of you can do on your dates. She may not be interested in sitting home and watching TV (or Netflix on the laptop or whatever) because it's free. You'll need to put some real effort into setting up hikes, visits to parks, going to museums on the free days, going to that cliff top lookout that overlooks the town at sunset, cooking a nice meal at your home and so on to show you're willing to put effort into the relationship, you just don't have cash.