Efficiency means how fast you do something... how should how fast they make the computers affect how expensive they are? It only affects how many they can make in a given period of time!
The early personal computers had lots of discrete electronic parts -- transistors, resistors, diodes, capacitors, etc. The circuits were designed with pencil and paper, very expensive in terms of engineering labor. The parts were a non-trivial expense. Then they had to be assembled by hand, also a non-trivial expense. Then there was all the qualification and testing. All those costs added up to a fairly high producer price per unit. The parts which were integrated circuits were also pretty rare, adding even more to the cost. Then they had to develop firmware in ROM, the power supply, display adapter, and other ancillary systems. There was a shortage of RAM, making it the single most expensive element of many computers.
[forgot to quote any supply/demand "arguments" but I'll answer anyway]
As for supply and demand, you asked why something with lower demand than now would be higher price. Primarily it was because low volume amortizes fixed overhead costs (office space, facility, executives, sales, distribution, service) across fewer units, making those elements of the price affect the consumer price more than they do today being spread across millions of units.
Don't bother asking for evidence BTW... My evidence is based on personally witnessing it and participating.