I see it this way...the things I can fix, I do fix, the things I can't do myself I pay others. The things I won't take on are electrical, mechanical, and plumbing...other than that I am pretty good...

On a serious note to a serious topic, if I can see it done the first time, I am usually good-to-go the next time around.
I don't know what's involved in a HVAC gasket change, but if it involves getting on the roof, and removing stuff...

I would pay someone else the $200.00 or so bucks...of course we are all speaking of the reputable service center or mobile tech who can and will take responsibility for THEIR work.