Not expensive to rent for 1day or half day and if you get the yestrebets pass even cheaper. Think Boro also have cheaper passes at weekend as locals go up not sure if they still do this though.
On a Saturday just hang around near the gondola and look for someone selling their pass. Worked for me last year and got one half price. Or just buy one.
No Iastrebets lift pass for the whole season.
Buying or selling list passes after purchase is illegal and if caught you could prolong your stay paper pushing and filling forms rather than starting the trip back to airport.
Borovets is best for last/first day quick skiing.