As a rule, it is not wise and even considered a bad method to include your salary expectation in a cover letter.

You want to be in the best power position discussing/negotiating your expected salary only after the company decides to hire you and gives you a job offer.

During an interview, however, they may ask you about the salary range and there is a way to handle this question – refer to the article interview question “what are your salary expectations”.  

Sometimes, however, there might be cases when a recruiter/employer asks for it after seeing your resume. It is their first selection criteria and you don’t have a way out of it.
In fact, it is always a no-win situation for you – you get into a form of stating a figure being either too high or too low.
 
My suggestion is that if the company doesn’t ask for it, then avoid revealing your salary expectations in cover letters.
You should first secure the position and get an offer before taking it any further.

However, what would you do if they do ask?  

How should you include Salary Expectations in a Cover Letter?

Actually there are 4 options for stating your salary expectations:
1. Give a wide range numbers, for having the best chance of matching the company budget.

2. Give a top range, aiming high, but ready to negotiate.

3. No number is given, stating “willing to negotiate salary after knowing I’m a strong candidate.”

4. No number is given, indicating ” flexible about salary, consider it as only one important factor of the overall job aspects.”
I wouldn’t go for options 3 or 4 – depend on the position you are applying, you don’t have any power key in hands before the company get to know you.  

You can combine two options and include your salary expectations at the end of your cover letter.  
For more information, refer to the articles:
Sample cover letter with salary expectations/requirements

Resume cover letter samples

Resume cover letter format