the cost and the time like you said.

you can do it your self for under 50 bucks in your driveway. depending on your year use what ever dex fluid you need. dex 3 for 97-03 dex 6 for 04 up) cheap stuff is ok really. walmart brand is cheap.

as far as when to service the tranny.....your late to the party already. depending an where you live, (hills or flat ground) you can start at 50,000 miles. 30,000 if you going up hills a lot or beating the snot out of it.