it depends, I would do many things at once and charge a standard price, instead of individual proces. Drives take no more than 2 minutes to install, so the priceing would be completely up to you.
I charge $360 for an xbox+X2 mod+120gb (or 160gb depending on what deals there are) HDD. $380 with a 2 color, painted case. So I guess it'd be about 60 to install a chip and the drive.
Oh yah, And I keep the original unlocked drives.
Edit, forgot to answer the first question. I don't pay more than 100 for a 120gb HDD, or a 160gb HDD. Just wait for the deals to come. For example, you can get a 200gb 7200rpm 8mb cache western Digital drive (ata 100) for 90 bucks+shipping here:
http://www.tigerdirect.com/applicati...p?EdpNo=423018
ata/150 here for 99+shipping:
http://www.tigerdirect.com/applicati...&Sku=TSD-200JD
Dunno if it is lockable though.