Q:

How much did the 6502 and Z80 cost?

It is said that the 6502 was cheaper than the Z80.
As of 1978, what were the actual prices of the two chips, in wholesale quantity?

A:

The MOS 6502 (1 MHz) was introduced in 1975 for a price of $25. Then in 1978 MOS agreed to sell the 6502 (1.79 MHz) and an IO chip to Atari for $12 per set (because the production cost was $4).
In 1977, the Zilog Z80A (4 MHz) was $65 for the ceramic package and $59 for the plastic version.

A:

The above answer gives prices for times close to the release of the chips and at dates of significant sales.  But note that prices were effectively in freefall, and that every quarter year saw significant reductions in price around this time.  The price at the start of 1978 is not necessarily the same as it was by the end.  What we can see is that within 3 years, i.e. by 1981, the price difference had effectively vanished. See the advertisement on page 77 of this magazine, which gives retail prices of various CPUs, including:

Z80 $6.95
Z80A $9.95
6502 $7.45
8080A $4.45

Given that this trend was ongoing, while there may have been a significant price difference between the two chips in 1977, would there still have been by the end of 1978?  It's very hard to tell without additional data that I haven't been able to find.

A:

Z80 Oral History - the gift that keeps giving...

Slater: Who were the primary customers early on, and what was the
pricing strategy? You want to talk about those aspects, either of you?
Ungermann: We went after the PC business as we know it today. And we
brought our prices down to allow us to do that very quickly. When we
came out with the product, we priced the Z80 at $200 a chip, and it
took us just a few days to understand from our customers that they
needed a lower price to be able to go after the market. However, we
also had a number of system companies that were thought that they
could get the IBM software to run on the Z80. They could crack a lot
of IBM markets, but that of course, didn’t ever happen, because the
Z80 was not designed to compete with a mainframe.
Slater: Were there
important embedded applications also?
Ungermann: Yes, there were many
embedded applications.
Faggin: Yeah, one of the first customers that
we had was Cromemco In fact, Roger Mellon came to the office and I
remember personally selling him one Z80 for $200, and Ralph didn’t say
it, but NEC was actually our first customer, and I think you sold the
Z80 to the competition. 
Ungermann: When we were packaging the product up for
shipment, somebody knocked on the door of the office and it was an NEC
guy with four $100 bills for two chips. And then right after that a
very successful Japanese computer company came in with four $200
bills. That was just the start of the copying.

