In terms of how often they come on the market for sale, Boober is definitely the rarest of the Hasbro softies. Now, that doesn't always mean most expensive. Once in a while you can get lucky, but typically, you can expect to find a Hasbro Boober in the $55 to $80 range (not including any shipping that might be involved).
And since you're a beginning Fraggle collector, here's a breakdown of the vintage Hasbro plushes, how rare they are, and what you could realistically expect to pay for them...
Gobo
easiest to find
$20 to $30
Red
also fairly easy to find
$25 to $35
Wembley
pretty common, but not as much as easily found as Red & Gobo
$30 to $40
Uncle Traveling Matt
pretty hard to find
$45 to $60
Mokey
about as difficult to find as Uncle Matt, most often missing her robe
$45 to $60 (without original robe)
$60 to $80 (with original robe)
Boober
the most difficult to find, sometimes missing hat & scarf
$55 to $80 (with hat & scraf)
$40 to $60 (without hat and/por scarf)
Doozers
pretty scrace, but they pop up once in a while
$20 to $45 each (there's two different ones)