There is a wide range of responses on the price of LPs "when CDs took
over" because it took about 5 years for CDs to take over, and LPs had a
broad price range, from the bargain bin cutouts making way for the new
format to the last of the new releases.
The first CDs were produced in 1982, but CDs didn't overtake the LP--that
is, they didn't outsell the LP--until 1987. By that time, new LPs were running
about $7.99-8.99, while there were plenty that could be had cheaper, though
not the newest releases.
Having bought Sheffield D2D LPs in the mid-'70s for $10 ea., my reaction
when CDs came out at $15-18 was, "well, at that price an LP would
sound better yet--if the money went into better recording, mastering and
pressing."
I'm basing my pricing on the vintage LPs I've bought over the past few years.
A few have had still been sealed and more have had the original shrink and
price tags on them. The good ones from the mid-'80s were going for around
$8.99.
As for the $3.99-5.99 range, I distinctly remember in 1969 when I bought
the self-titled Creedence Clearwater and Blood, Sweat, & Tears albums, that
the MSRP was around $4.99, but they sold at my local department store for
$3.19, and they had the best prices in town. Within 3-4 years new LPs were
going for $4.99 at Tower and The Wherehouse. In 1975-6 when I sold audio
equipment commercial LPs were around $4.99-5.99 and the audiophile
Sheffield LPs were $10.
BTW, adjusted for inflation, those last LPs in 1987 at $8.99 were about the
best bargain--they translate to $16 in 2008 dollars, whereas a $5 LP in 1972
is equivalent to $24.53 today. So the price of LPs today is about the same in
real dollars as what they cost in the golden age of analog.