My son's 2000 Honda Accord needs a new transimission. What should it cost?

Evidently these transmissions aren't that great and have been a problem. He has had an estimate of 2,900 dollars which seems a little steep. Is that a little high? Also, when it is rebuilt do you recommend a transmission cooler on the rebuilt transmission? We want it to last. Evidently some people have had to replace these transmissions every 60,000 miles. This one lasted 125,000 but I had the fluid changed every time it was dirty and it just got dirty quick. Why? Does the transmission run too hot, or is it made of poor materials? This is an excellent car, I turned it over to my son at 105,000 so he'd have wheels for grad school. We had hoped after all the maintenance done on it that he could get a few years out of the car but it only went 20,000 mles more until the transmission starting slipping--which is intermittent not really bad yet. Please advise, thank you.