For those of you wondering, here's some MSRP prices dug up:
1973 GT-150 $1,469
1973 Mercury 850 $1,493 (without controls or prop)
Yup, the motor was selling for more than the boat. With controls, trailer and set-up, maybe a dealer special of $2,995? (About the same price as a 1973 Mustang convertible.)
** Special thanks to Bob at MercGuy.com and Chris at OldMercs.com for the Mercury MSRP selling price back then. **