CR-Miko,
You really can't put forward a story like that and not expect people to react. By your own admission not asking the price in advance was a big mistake. Others have harped on that but there is really no need for anyone to belabor that point since it is so self-apparent. None of us were in the vehicle with you so it is really impossible for anyone besides you to say truly bad this guy's driving really was. Others are right that virtually everyone in CR seems to drive horribly, but I'd be willing to allow that you could be right that this guy's driving on this trip was even worse than that already poor average. But that still leaves the issue of how much he charged you which you yourself brought up. Are you saying that if it weren't for the bad driving you would not have any complaints?
So lets put aside the issue of how bad his driving really was or was not and like at the price issue. You were expecting $300 round-trip? I've never personally used any private driver myself but I do have some idea what they charge and even I know that would be an incredibly (impossibly) low price. I think the MadGerman's math is a little off. It is 307kms from SJ to Tamarindo or just under 200 miles. Let's call it 200 with the various side stops in Heredia and such. An average passenger van still gets about 15mpg (or more hwy) but lets say he gets only 12.5mpg. That comes out to 16 gallons each way. At $5/gal, that comes out to $160 for gas alone roundtrip which is a far cry from the $300 that MG came up with. The typical tico makes only $25/day but lets say an experienced (and safe?) driver makes twice that or $100 for 2 days. Now we're up to $260 and when you factor in maintenance, depreciation and insurance(?

) your initial $300 estimate does not seem so unreasonable. The problem is that reason has nothing to do with how they price things for gringo tourists in CR and typical driver prices are often double what reason says they should be. That is just how the market works.
Personally, I've never understood why ANYBODY even bothers with these guys particularly given the ridiculously high amounts that they charge. Even at the $300 rate that you estimated, you could have instead flown with Nature Air in less than 1/4 the time you SAID it took you for $190/rt (or even as little as $156 for their special fares that are usually sold out) and still had more than enough money for all the local cabs you could possibly have needed in Tamarindo. Oh, but you say there were 2 of you? At $380 for 2 people, it is still much less than the $550
normal market price you paid and you wouldn't have had to spend 7 scary hours on the road. Oh but you say you wouldn't have been able to leave when you wanted? For $170 savings I'd be more than willing to work my itinerary around the usual flight schedules. After all, does it really matter that much if I go to the MP in the morning or the afternoon or whether I sleep in or get up early and take a nap later in the day?
But I'm not even one for flying. For ~$133 ($110 if you took their afternoon departures) the BOTH of you could have used Interbus which (even with local cabs) is less than half of even your $300 estimate and less than 1/4 of what you ultimately paid. For that sort of savings I could definitely adjust my schedule and make a few stops along the way to pick up other passengers (which is what you did anyway).
I won't even go into how much you could have saved by simply taking the bus, which is my own preferred mode of travel within CR, because I know it comes with trade-offs that many if not most guys aren't willing to make, but the savings from that would have been about 95% off of what you paid. So I really don't think you have much basis to complain since you chose the method you did. And I'm not even convinced that your experience with this particular driver is appreciably different than what you might have had with someone else.