The cheapest I can do it for is as follows (inc. shipping)
$98 regulator with solenoid
$45 adaptor
$40 per sodastream bottle new (around $20 second hand on TM)
Diffuser, bubble counter, check valve etc anywhere from $20 up to $50 usually
Refills cost around $12 and last a 60x30x30 high tech tank around a month.
How big is the tank you want it for?