Ideally this is why it's good to work out what costs might be incurred in a new job (transport being the obvious one) before accepting, however this isn't always feasible or foolproof (after all life can happen, prices can rise etc) and obviously it's a bit late for that advice for you now!
So what can you do?
See if there any alternative ways of getting to work - not sure what distance you are covering (or what method of transport you are currently using) but would it be feasible to switch to a cheaper method of transportation?
- If you're currently using cabs or a car would it be possible/cheaper to swap to the bus or the subway?
- If you're currently using the bus/subway would it be possible to cycle or walk instead?
- If you're currently using the bus/subway have you looked into the various metro card options? Multi-day passes often work out significantly cheaper than paying for fares as you go. If you're a student I think you can even travel for free?
I appreciate that this may well take longer than your current route but if the savings meant you could keep the position going it may well be worth putting up with it as presumably you won't be an intern on $100 a week forever.
See if there are any other areas of your existing spending that you could make savings on, in order to free up some spare cash? Again this would hopefully be temporary until you've completed your internship and can start earning more money.
Asking your employer if they can provide any additional assistance financially. I wouldn't make this one your first move, if nothing else they will probably ask you if you've tried Options 1 or 2 first! However if, having exhausted the possibilities of the first two options you're still struggling to afford the transportation costs then it's certainly worth asking them - after all if the only option you have left is to quit what harm can it do to ask? I'd try something like:
I'm finding the costs of getting to the office too high, I know I should have checked this out beforehand but I wasn't expecting it to be this much. I really want to stay here as I'm enjoying the internship and I feel that I'm really learning a lot. I've already done x, y, and z to keep the costs as low as I can but I'm struggling and I'm concerned that I might have to withdraw from the program. Is there any way we could look at increasing my compensation, even by $x a week to enable me to stay?
This shows that you aren't just looking for them to solve your problems for you, and that you have been taking steps to try and resolve it yourself first and this demonstrates good maturity (which will hopefully make them realise you aren't just money-grabbing).