If you're buying a collectible as an 'investment', it might be worth that to a select global crowd, who knows.
If you're buying optical performance, though:
Pentax Full Frame K-FF body: $1700 - $3000 (your guess)
Pentax DA or FA 200mm f/2.8 = $1000
= 135mm f/1.8 in aps-c terms = $2700 to $4700, and we're probably talking more MTF, less CA than that 135 1.8 on K3.
And you get a free FF body for that $2700 - $4700!
(personally If I were to buy right now I might buy the FA/DA 200 2.8 used for around $400 - $600 to pair with A7R, it would be MF too.)