Do you already have a guest list? Is it friend-only, or family, too?
I had told a college friend that I would love to have a baby shower for her whenever she got pregnant, and right after she found out she was expecting one of her very good high school friends immediately volunteered to do the shower. However, she was awesome enough to recruit me to help along with my friends' sister (it was a friend/family shower), so we were all able to help out proportionately. There is nothing wrong with being honest that you cannot fully fund the shower (especially if you did not offer to host it), but if you feel you need to, see if you can enlist one or two other people who can and want to help you out as co-hosts.
We had this friends' shower in a nice, enclosed park lodge (it was yucky-weather season) and we all pitched in for decorations and family and friends all signed up for food. Her aunt is a cake decorator and made a cake and cupcakes. You could ask friends, co-workers, and family if they have any baby shower items, like decorations, as they get used once, stay cute, and are perfectly fine to re-use (if they fit your chosen "theme").
A park shelter or lodge might be cheaper than a restaurant, but you should call and check prices. A park would let you be as thrifty as you like for food and drinks and take as much time as you want, whereas a restaurant would probably be more limited for time and might require minimum food purchases, but I am not sure.
Anyway, I would send out a quick email or note to a few "top" friends on the list asking them if there are any areas they would like to help you out with in order to throw your friend an awesome shower. Every little bit helps, and it really is a big committment for one person to host one that large. I don't think I even know that many people closely. :)
Oh, just another thought--do you or one of your friends' family members go to a church with a meeting or fellowship room you could use? That would be less expensive, possibly free.