In most normal cases, you should not include your salary expectation in cover letters unless you have very good reason for doing so. This may be when a recruiter contacts you after seeing your resume somewhere, such as an online job board. In this particular case, then it is okay to reveal your salary expectations early, in order to avoid wasting your time and theirs. However, barring this one difference, you should just write the cover letter as you normally would.

Writing salary expectations without first being asked for them can possibly make you look like someone who: a) jumps in without thinking or b) is desperate. If you include your salary requirements on the cover letter, you take the chance of the figure being either too low or too high. In the first case, the company will save a lot of money, particularly if you have the skills that the employer is looking for (and you will be giving your skills away). In the latter, you will potentially spoil your chances of being called for an interview if the figure is too high, yet you are willing to negotiate in other areas.

If you genuinely feel that you should include your salary requirements, then by all means do so. However, you need to do this with a bit of finesse; you have to do it in such a way that you won't be pigeonholed into a particular figure. One effective way to do this is to employ the "range technique". You'll have to do some research and provide a range, beginning on the low end and ending on the high end. You can write something like, “Here is my salary expectation as required, which I believe will fall into the prevailing salary range for the position”. You should also indicate that you are willing to negotiate. Writing a full paragraph or not sufficiently researching the prevailing salary in the industry may create an odd impression, which will probably work against you.

Providing your salary requirement is not a guarantee in itself of anything, let alone a hiring decision, so don't ask for a figure that is obviously higher than the industry standard. However, asking for too small a sum can fail to project you as a mature professional with overall awareness and one having good negotiating skills. All positions advertised will have salary ranges predetermined and asking candidates to specify them is really to find out whether suitable candidates fall within the range. The final salary depends on how much a particular employer is willing to invest in you, your history and how you can contribute to the company. Nonetheless, the final figure still rests within a predetermined range, barring extreme cases where you can strongly justify your case.

In general, you should be reasonable with salary requirements and take care not to let the entire cover letter circle around it alone – if you decide to include it at all. Be polite and matter-of-fact when writing it, as the company could be asking because of difficulties the company ran into due to someone over-qualified who asked for an out-of-the-range salary.

One thing that you should remember when faced with the "salary requirement question" is that companies do expect you to negotiate. Don't disappoint them.