Our chapter was solely a COB chapter for years. One painful year, when Quota was only 9 we got 2! I was COB chair and luckily I was able to recruit us 11 wonderful women after Rush. Every chapter at our school is dependant upon COBs, since Formal Recruitment never has many women go through, quota would be between 10 and 20, but total is 100.
What worked for our chapter in our school (a HUGE school with a small Greek population) was ideas that didnt involve straying too far from campus.
There was one place off campus that was popular- it was a pizza/bar/resturant that we could rent out the top floor for COBs. We'd invite the PMNs to our house, then walk the two blocks to the resturant. We'd order pizza and soda and it would literally be a party with the PNMs. Of course it was free, and we got women who just wanted the free food (we'd advertise it in the newpaper the week before) and turned out to love the sisters.
Another fun idea was to have Sex and the City night at our house-- we'd get non-alcoholic wine or martinis, and cheese, and watch DVDs of the show. But we wouldn't put in the show right away-- we'd wait until everyone talked for a while and relaxed. Most people were so interested in talking that most didnt watch the show, but it gave it a cool theme that interested PMNs.
other ideas: having a dessert party at an Italian resturant, making ice cream sundaes or taking them out to ice cream, and we once took them out to ice skating.
our budget was always tight, but I got a good percentage of it, and sometimes charged sisters, but often we'd spend thousands on Formal rush to get relatively few women, and then spend $50 on ice cream to get a lot