Wouldn't it be better if you put that $5/month and let them know upfront copy/prices are not set - they are meant to communicate what that area is for and how much space is dedicated to it.
What if you had in your mind that the space is for $5/month, but used lorem ipsum and designed around that.
Then the client hands you copy that goes '$5/month, $50/ year, Save $10 if you subscribe annually / 20% off.' This will likely to have an impact on your layout and spacing, since you just used 'lorem ipsum' in the wireframe step.
There are also times (like in a small company) when the person reviewing the wireframes is also the person reviewing the final copy, so if you put real copy in there the meeting will always get sidetracked into a discussion of the final copy unless you use some type of filler.