Choose an image that you like from an agency like Alamy (Alamy.com).
Then explain the kind of usage that you are interested in, and ask for a price quotation.
That is the price that I would quote to my client, or something less if you know your client would not pay that sum. Remember you get all the proceeds, while an agency must remunerate the photographer and itself (fifty-fifty is usual). There is no agency work involved here (besides the quotation...) so you could even ask half of what the agency asks for that usage and remain a happy camper.
You have to know how many copies of the CD, how many copies of the posters, and what kind of "promo" on the internet. Have those details clarified by your client before asking for the price quotation to an agency. Avoid "microstock" agencies of course. Alamy should be fine.
There probably is some "bulk discount" involved: CDs, posters, internet promotions do not simply sum up.
You will retain the possibility to license your image for other commercial uses unless you sell exclusive rights (for a certain number of years, or in perpetuity). In any case, even if they buy no exclusivity, you will not be allowed to license that image in the same industry (e.g. for another CD or posters of another band).
Remember when you talk with your clients you don't "sell" your image, you "license some use" of it.
PS Before asking a quotation you should familiarize yourself with concept such as Rights Managed, Royalty Free etc. You will find an explanation on each agency web site.
If you are so good at making pictures that people ask to license them, I would strongly suggest to open an account at Alamy (or other agency) and begin selling through them.
