next day pill, plan b how much does it cost and can I buy it for my wife?

If you are in US I would try Planned Parenthood. They are reasonable although dont know how they do it with you buying it for her since they give it to you there, no perscription etc.

Most stores which have a pharmacy counter and that dispense prescriptions will sell it.

For example, in the UK, Boots stores, Superdrug and many independent pharmacists will.

There is also the option of a family planning clinic, and even your GP/doctor.

Here it is possible to get it free with certain circumstances, however your wife will need to discuss with the pharmacists herself, as they will need to ensure that she is not allergic to any of the ingriedients, and that she understands all the T&C about the pill itself. It can be bought for around £20 here in the UK, but again, I must emphasize that your wife must go and have the discussion with the dispensing pharmacist, they will not accept somebody asking on behalf of somebody else. They will however allow you to attend the consultation if your wife consents.