I wouldn't call them rare, but they are far more scarce than their larger cousins. Seems the smaller the model, the fewer they sold (or the fewer were kept from the scrap yard, or both). I don't see them fetching much, and the earlier and cleaner, the more $. The 515's with the solid rubber front tires are a tough find. They share transaxles with Braodmoor's and Yeoman's, so if a replacement is what you need, they can be had super cheap. You ahve some neat memories with it, and it sounds like it may get it's second wind? =]
I watched them come and go for a while and finally gave $200 for mine, running, with an unused deck. The tractor is nearly new, but missing the seat cover and deck chute.
I've since collected the rear counterweight and doughnuts, OEM wheel weights, Dozer, Tiller, Revitalizer, Grader, and pile of NOS accessories (HiLo, lift kit, Hubcap kit, and deck chute). I've seen a couple vac systems, but they are just too large and impractical. If I could just find a 27" blower and a seat cover.....it may become the most expensive Serf ever! (but it will be worth it, when that kid starts making memories like you have!)
It sits for weeks or even months, and always starts on the first pull...I LOVE it, and am putting the package togther for my son, when he's old enough to enjoy it.