There really are alot of factors, if you have pics that would help out alot. If by garage kept and in excellent condition means it hasnt been sailed at all, and has been sitting around for the better part of 30 years, then you could pay up to $2000, even for a 1973 boat, whereas if excellent condition means good for a '73 boat, I wouldn't pay more than $600.
For me, trailer is also really important, and you really need to watch their integrity. I had a '78 H16 on a trailer, and got it in good to great condition with lots of extras for $500 flat. Registration in CA added some on, but then what really annoyed me was the first time stepping the mast, the front right steel crossbar support on the trailer simply bent down until it was touching the ground. A 4x4 all the way across fixed it, but looked horrible.
Now for example, I just purchased a '74 H14 in amazing condition, i.e. no soft spots, no tears in the sails at all. The guy's wife wanted him to get rid of all his boats, so he gave me an entire '85 H16 along with it, minus the trampoline. (He had really beaten up the hulls, but the sails were great). The best part was, he had way too much money, and just wanted to get rid of them, so I got them both for $440.
So, it really depends on the case. These are just my better stories of Hobie purchasing.
Without pics, I would guess $750. It really depends on the circumstances.