You should look at oldcomputers.net for this info.
Radio Shack TRS-80:
basic unit with 4K RAM and B/W video monitor: $600
basic unit with 16K RAM and Level II basic, monitor: $1000
($300 Expansion Interface required to add more than 16K memory)
basic unit with 16K RAM, Level II, Expansion Interface, monitor: $1300
5.25" disk drive (requires 16K, Level II, Expansion Interface): $500
Commodore PET:
PET 2001 (4K RAM, built-in B/W monitor): $800
(not especially upgradeable memory-wise)
PET 4032 (in 1980; 32K RAM, built-in B/W monitor): $1300
CBM 8050 dual floppy drive (in 1980): $1700
Apple II: (in 1977)
Basic unit with 4K: $1300
Basic unit with 16K: $1700
Disk II floppy drive with controller card (1978): $600
Atari 800: (in 1979)
Basic unit with 8K: $1000
(includes slots for 3 optional 16K RAM cartridges)
810 floppy drive: $600
Exidy Sorcerer: (1980)
basic unit with 8K RAM: $900
basic unit with 16K RAM: $1150
S100 Expansion Unit: $420
Video Disk unit (B/W monitor + 2 floppy drives): $3000
Note that prices came down over time, especially due to decreases in RAM prices.
So, I'd say that there was something of an "Apple tax" even back then, but it wasn't really so much. When you considered how much expansion capability you got with the basic unit (which for other systems was either an add-on or simply not possible), it was actually a good deal.