I got up the nerve to ask him for an invoice. He sent a list with about six items that totaled over $200 --
1) 3/4in lead free ball valve $19.80
1) 3/4in copper tee, mip, dip, and two 3/4 90 $23.76
1) thermo expansion tank $42.62
2) 3/4 x 24in water heater flex $47.08
1) water heater strap kit $35.18
1) 1/2in gas cock w/18in flex $32.83
1) 4in adjustable ell $7.24
1ft 3/4in copper pipe $1.50
So, while I was right in seeing that the expansion tank wasn't worth $200, he had been honest. Of course if he had itemized these things on the bill the question would never have come up.
Also, what had made me watch my guard is that he rounded UP the time he spent to four hours, when it had been 3 hours 45 minutes including the 15 he had spent sitting in his truck preparing the invoice. That ticked me off and made me feel he was trying to take advantage of me. Btw, I'm female, and my spouse was nowhere around...
Anyway, water under the bridge. Next time I deal with a professional in such a situation, I will be very clear about the start and finish time, sending a text to confirm it. Nobody wants to just give away $40 extra for rounding up. If he were so rich and generous, he could round down that much.
Thank you to all who responded.