Bennetts are an easy install on your boat if you're handy, and yeah, about $500 for a kit. They make a nice product that works very well. Todd Bennett hosts a section on THT if you want to browse.
I keep telling people to try the PERMATRIM first if you're just looking for bow trim. I have both and pretty much only use the tabs for 'thwartship leveling from uneven loading (people's butt's) and prop torque. Andy at SIM is the north American importer, he also has a section on THT if you want to read up.
Tabs are nice to run the bow down when slogging heavy chop. You can cut the power back and run at 12-14mph with the bow down and stay on plane. The Permatrim is about 75% as effective as the tabs, but it does it's job getting the boat up on plane. For cruising speed on up I don't use the tabs, just for slower speeds where the bow wants to ride high.
As far as an install, a 'dealer' or marina is going to charge you full-boat list or more for the parts, and $65 to $100 an hour to do the work, and they take their time. Some are good tech's, some not so good. That's why I do everything myself. So they're probably charging you $800 for the kit and $500 to put them in, allowing themselves a day of labor. What a joke.