I would either get something secondhand (old good shimano 600, 105 etc) or Tektro make good cheap brakes which should leave you enough to get a lever, cable and housing.

From what you describe about a brake lever your looking at either: a BMX brake, a cyclocross inline lever or an old MTB brake. Hardest thing is ensuring the clamp on these brakes work with your bars which I believe on the Pista are 26.0.

Again second hand is your best bet as otherwise you're going to be out 15 bones plus. Its going to be hard to find a BMX brake which works with these bars but the old DiaTech MX brake levers are adjustable enough (though pretty flexy), maybe some of the snafus?

Talk to your LBS and maybe look around for a shop dealing in used parts.