Abortions can be done up to about 4-5 months into a pregnancy, and it takes about a month for everything to work right again after, so from an economic perspective, as long as 6 months of contraceptives are cheaper than an abortion, I'd agree that contraceptives are preferable. (Again, this is from a purely economic perspective.)
The average abortion runs about $400. Condoms can be had for around $0.15 each. She'd have to average greater than 14 times per day for abortion to be cheaper than condoms for birth control. (Although if someone is averaging 14 encounters per day for greater than 6 months, I'd say he/she has bigger issues than deciding on a method of birth control)
Depo-provera runs $70 to $150 + exam fees for 6 months, so call it anywhere from $70 to $300, and usually closer to the bottom of that range. IUDs run $500 to $1000, but last 5 to 10 years, so that's about $50 to $100 every 6 months. A 6-month supply of birth control pills runs about $90 to $300, and usually closer to the bottom of that range.
Given the regular doses of BS I get from my sexually frustrated friends, I think the world would be a better place if we encouraged people to bump uglies, and regularly.