Yes, well free instruction helps the cost. If you can get that, go for
it. I flew an hour and a half with an instructor and it was more per
hour than renting a 172 with an instructor. The tows weren't cheap, I
had to rent the glider and pay the instructor. I believe the short tows
I had were about $30 each and I had three of them. The instructor was
about $35 an hour and then there was glider rental. But it would only
take me about 5 such lessons to add on a glider rating on top of my
airplane rating. Also, did you include the club membership? Your $1600
seems awfully low, if you can get it, it is, indeed a good deal. Go for
it!