In the mid 1970s Soviet High Command decided to re-equip Group of Soviet Forces Germany (GSFG) in short order with the T64A. The plan for re-equipment was 6-7 divisions a year (around 2000 tanks), so that the GSFG would be completely re-equipped in 3-4 years. T-64As began arriving in 1976 (and were mistaken by Western intel to be T-72s – hence the beginning of the myth that T-72s were the main tank of GSFG).
At first re-equipment went quite quickly, helped by the stripping of T-64As from divisions in the interior USSR, and in 1977 six tank and motor rifle divisions were re-equipped. But the single Kharkov plant producing T-64As (Leningrad insisted on their T-80 and Nizhny Tagil on their T-72) couldn't keep up the initial pace, hence many GSFG were still equipped with T-62. Uralvagonzavod (at the time producing T-62) was slated for production of T-64A in 1970. Kartsev initially hoped to develop a rival to the T-64A, and after being refused by Moscow, he proposed a "mobilization" version using a normal diesel engine from the T-62 (instead of unreliable 5TDF engine), as a low-cost alternative for mobilization.
Uralvagonzavod developed a new auto-loader and introduced improved V-45 diesel engine. T-64A hull and turret were reconfigured to adopt those, which resulted in Object 172. A state decree on standardizing the T-64A gave Uralvagonzavod permission to further develop the Obiekt 172 which took it even further away from the T-64A configuration by permitting incorporation of the new suspension from the Obiekt 167 (which was improved T-62). This version was designated Obiekt 172M, which will later be accepted as T-72.
T-72 in the end didn't turned out as cheaper, but was at least nominally more reliable and better adapted for mass production. Another thing that always separated T-72 from T-64B (not T-64A) and T-80B was lack of proper automated fire-control system. That is why latter was consider "premier" tanks and were used by Group of Soviet Forces Germany (GSFG) and mostly elite Guards divisions (although there were some exceptions) while T-72 was kept in Southern and Northern Group of Forces (respectively Hungary and Poland) and most of interior USSR. This also resulted in T-72 being widely exported.
There is still a widespread perception of Soviet military system as highly centralised and decision-straight system, but much as Western corporations fought over military orders so did "clan wars" between different design bureaus in USSR resulted in three different, yet similar tank designs, which in the end was a fairly inefficient solution.
The T-72B and the sub-series it spawned represented a very significant step in the evolution of the T-72, with the introduction of bulging armour in the turret as well as in the hull later on. Still, other than armour, I still find it inferior compared to T-64B and T-80s, but game price calculator doesn't take into consideration the difference in aiming speeds which is how we simulate automated fire-control-system.