I apparently have a leak from the second floor roof. Roofer says it is the plastic boot which is about 8 years old and probably has a crack in it. The cost he quote to replace al 4 boots was about $900. That seems high to me. He said he would replace with an aluminum boot. Thanks.

Get another estimate or get several estimates for this repair.

His prices are very high unless there is something about your house that will make it very difficult to replace them. Boot jacks come in many materials and what he suggests should be fine but for around $400.

Yes, I will agree, I have replaced my own, and depending on where you are but the 800 is way to high.

OK this is fairly simple to do I charge 200 a piece but as someone said it depends if these are in an area which would be difficult to get to

I am a woman and a few years ago I replaced the boot on my exhaust vent. Cost me a few dollars, for the boot and caulking.
That roofer is trying to rob you, don't fall for that stuff, unless you can find an honest woman roofer lol.
Anyway, go to home depot, you will have to know the pitch of your roof, and they will help you free of charge.

Since the rubber boot surrounds typically only last 5-6 years, there is a new product on the market now that fits over your existing boot and pipe and seals it. It is apparently guaranteed for the life of your shingles, but I've never used it : http://www.permabootstore.com/