Hey James,
I reported last October that my first visit to the Blue marlin was with a female "wing-man." She's heterosexual, tall, blonde and stacked. Nobody in the bar was outright unfriendly, although she got a few hostile stares from the working girls while in the casino. Phuck 'em.
I agree with the poster above who stated that a lot depends upon the attitude of your female friend. Mine was terrific, just had a centennario and coke and took it all in, smiling. The waitress was very cordial and service was good, nobody gave anybody any trouble. But...
Imagine that your friend suddenly gets religion, or gets feminist, or political. What if she actually considers turning a trick or three to help subsidize the trip?

Lots of variables, and only you know whether or not your gringa friend will be cool or not when faced with a couple dozen women who will undoubtedly look better than she. I think a better option would be to follow Shy One's advice above; go to the Key largo on a Friday or Saturday night and dance/party the night away. Lots of couples, tico and tourist go there; the band we heard there was absolutely stellar and we had a great time.
And don't get pissed if some drunken gringo propositions your friend...it could happen!
Accomodations: I love the Don Carlos, very near the Hotel Castillo and Sportsmen's Lodge, and right across the street from New Fantasy. You could even arrange a therapeutic 90 minute massage for your girlfriend through the hotel desk ($35.00 last year, a great price, no sex) and then use the time to go get yourself some therapy across the street or three blocks away.
Price? How cheap are we talking? At $45.00 - $70.00 + tax per night (average price for low-season rooms in the area), you'll find a half-dozen nice hotels. Less than that, I can't say. Check a variety of travel resources:
http://www.letsgo.com
http://www.frommers.com
http://www.fodors.com or other non-mongering websites. The Let's Go" site is geared toward budget travel, but you'll find reviews of inexpensive lodgings at all three sites. Good luck.
You and your girlfriend might enjoy Gringo Gulch, you probably won't get hassled, but everything depends on her attitude, limits and values. Again, good luck.