Yes, air travel was indeed extremely cheap in the Soviet Union. Almost like Ryanair
Anyway, about the engines on the Tu-144LL. I read an article in Tehnikamaailm (a tech magazine in Estonia and Finland) a while back about these test flights. If memory serves me correct, around 20 test flights were commenced.
But the trick was, when they tried to find a suitable aircraft, they could easily find such airframe but gathering four working engines was a real challenge. I can't remember how they solved the problem, but I'll try to look up the magazine later and share.