If you are really cheap, beg or even borrow an old used saddle from the junk parts box of your local bike shop. Or you can scrounge around at the goodwill and buy an entire bike for $10. Maybe a takeoff from a new bike when a customer upgrades. Dumpster diving, junk yards and garbage dumps are a good source too. Really when you get right down to it, if your budget is $40, borrow the extra $30 till you can pay it back or sell something you don't use. Maybe even offer to work a second or part time job for a weekend or something. Theres lots of ways to get an extra bit of money.
A good solid leather saddle is worth the extra money and if you can't buy one right now get one of the others in a womens width model, they will be closer to a B-17 in width. I've got three or four junk saddles I would give you if you lived in my area just to get rid of them.

Oh yea.....a $15 Brooks saddle cover will fix your present cracked saddle just fine
and you can use it later when you can afford the Brooks. Or you could make a cover out of scrap canvas, leather or any suitable material for now.