Economical? You must be feeling flush. There's a reason they're giving them away, because they'll burn 120gals of Jet A1 per hour, all of them, minimum.
You're options are:
Jet Provost
Fouga Magister
Casa/Hispano Saeta H200
Iskra
L-39
Moraine Saulnier Paris Jet
Out if these the Magister and the Provost are probably one of the cheaper to run as parts are plentiful. The Fouga is supposedly a very docile aircraft that won't bite.
Slightly more high performance:
L-29
Folland Gnat