I want to buy suppressor from a dealer in another state and have it sent to my SOT. My SOT wants to charge $100 to do the NFA paperwork. Is this a reasonable fee? Sounds excessive to me. So basically my $200 buck can will end up costing me $500!

I would shop around and see what others charge.

ETA: You may also want to ask your local SOT what he would charge for that same suppressor if you bought it from him, unless it's something that you know is backordered for a long time.

Shop around and see if that's the going rate in your area.  My first 2 transfers were $150 each...  Yes $150 for a transfer.  The aren't a lot of NFA dealers so in order to not have to drive across the state, you do what you got to do.  There are 2 new dealers that I know of, 1 guy is only 10 min away so I'm curious what his x-fer price is.

Things are expensive in Vietnam.  

$100 is high but not grossly so. My dealer charges $50 but some of his competitors in a 50 mile range charge $75 ~ $125 and still have enough customers willing to pay that to make it worthwhile.

If your dealer feels people are willing to pay it without losing enough customers to be profitable, so be it.

Call around to all the dealers in your area and find the one charging the least (that you would still trust!)