Some thoughts:
1) srilm, thank you for that fascinating information. I really mean that. I really love learning about arcane stuff like that.
2) I'm not sure where MG and crookedcr are getting their figures but the HIGH season rates on Nature Air from SJ to Quepos are more like $67/pp each way anytime of day, not $85-90 or $100. Also, as far as I know park entrance is still just $6 for gringos and $1 for ticos (and maybe another $20 for both of you if you want to hire a guide). Still their main point about it all adding up (r/t for 2 people plus all the costs once you get there) to a fairly significant sum is well taken.
3) Quepos would be the least expensive of the regularly scheduled domestic destinations. Tamarindo or Arenal or anywhere else would be even more expensive.
4) where you get the discounted fares is during the off-season 4/21-11/30. So you might want to save this activity for another trip. OTOH, there are also fewer daily flights during the off-season so your choice of departures will be more restricted as well. For example the 9:15AM departure to Quepos is only $39/pp. The cheapest return flight is $47/pp. So $86/pp r/t (before any other ground expense) might not sound too bad, until you realize the last return flight is at 12:55 leaving you only 3 hours there. Of course you could also take the 3PM bus back to SJ and save even more money (and still giving her at least 1 time up in the air, which if she hurls may be all she really wants).
5) The best flying weather is earlier in the morning and the only way to make it worthwhile as a daytrip is to leave as early as possible, but have you ever tried to get a chica to move fast early in the morning?
6) This is a commercial flight in a small plane probably with others (not to mention the pilot), so I wouldn't really expect to be able to join the "mile-high club" and the only "hummer" you'll probably get will be from the throb of the engines.
7) You could look into booking a private charter to just take you up for a short flight around the central valley. And you might even be able to get the pilot to look the other way while you and your amiga joined the mile high club (whe would probably be more interested in enjoying the views). However, I'd bet that would be even more expensive than putting together a whole day or even overnight trip to Quepos on a regularly scheduled commercial flight.

Crooked is right that a half a day at the beach is probably enough, but there ARE other things to do and see there as long as you're going to all that expense to fly there. And, I'm sorry hankkn, but flying to Quepos for the day and then paying $150/hr for sex with some skanky ho when you can just wait until you get back to SJ and get some much cheaper and better quality seems just plain nuts to me. OTOH, Versatile will have a chica with him. One that will probably be eating out of his hand for treating her to an experience she has never had before. And he will have to pay for a hotel room somewhere whether he stays over for the night or returns to SJ. So why not milk the trip for all its worth and stay overnight with this girl.
Conclusions:
1) If you want to do this during high season, when ticket prices are at their highest, you could take either the 6AM or 9:30AM flight for $67/pp(6AM would be better to insure you could get in the park and to allow more time there). There's a 2:10PM or a 5PM return flight for the same price but if you want to keep costs down (or she doesn't react too well to the flight out), you can catch the 3PM return bus for less than $5/pp. Add it up: $67/pp for the flight out, 2x$6 cab (each way) between the airport and the park and a similar amount in SJ to get to the Pavas airport, $7-10 to get in the park and $47-50 for food, drinks and incidentals and $5/pp to take the bus back and the price adds up to an even $125/pp which is not TOO bad for a day trip. Fly back and the price goes up to ~$185/pp (or $370 for the both of you which is even more than MG's original partial estimate). BTW, this would allow anywhere from 5-13.5hrs in the Quepos area.
2) If you do this during the off-season, say next month, you could take the 9:15AM flight for $39/pp and return the next day at 12:55 for $47/pp. So r/t airfare would cost only $20/pp more than o/w would during the high season (or only $15/pp more if you also figure the busfare saving). Other costs would be pretty similar. Okay, you might spend a bit more on activities to fill up your time, such as a zipline tour, kayak trip, an evening sail etc. But you'd also be getting so much more time there and enjoying all those activities. And the rest of what you'd spend for hotels and meals would be fairly similar to what you'd be spending anyway back in SJ.
3) And if you do this as an offseason day trip and take the bus back, you get 5.5hrs there and the total cost for the day comes down to less than $100/pp