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Magnification is the quotient of the scope's focal length and the focal length of the eyepiece.
So, if your scope has a focal length of 900mm, and your eyepiece is 25mm, then the magnification is :
900 / 25 = 36X
But there are practical limits to the max & min. The max has two factors affecting it, the first is the size of your objective lens or primary mirror. A rule of thumb is 50X per inch of aperture. So if your scope's diameter is 114mm (4.5"), then the max magnification is 225X (assuming the optics are decent quality). BUT, the ultimate limiting factor, regardless of the size of your scope, is the atmosphere. Typical conditions allow for 100X to 150X, and it doesn't matter if your scope is 4" or 20". Good conditions will allow upto 250X. Exceptional conditions are 400X and over, but this happens maybe once a month.
There is another factor, but this affects viewing comfort, and that is the EP design. Plossl EPs were the bees knees of EPs, 30 years ago. They were expensive to produce then, but today they are chip as chips to make. They still are a good design, giving good image quality when made well. Their main draw back is the eye lens (the lens of the EP you look into, as opposed to the field lens which faces into the scope) gets progressively smaller as the focal length shortens, and you need to place your eye closer and closer to the eye lens until you need to look through a pin hole and park your cornea on the lens too to look into the thing.
Today there are much newer designs that also employ newer glass types. These new designs don't see the eye lens get smaller - in fact some new high power EP field lenses are bigger than that of a 25mm plossl. The eye relief (how close you need to put your eye to see the whole field of view in the EP) is longer too. Again, new designs today can be as easy to look into as a 25mm plossl. New designs also allow for a wider aparent field of view, where plossls give a 52deg AFOV, new designs give from 58deg all the way up to 120deg.
Price is a tricky thing though, but some careful research will find you can get damn good high power EPs for as little as $70 new (the TMB Planetary Type II models). Their AFOV is only 58deg (still better than a plossl) but their ease of use is akin to your 25mm plossl. They pop up every now and then here in the IIS classifieds, but are snapped up real quick. The 9mm is a real gem in my kit!
One little known gem is the Celestron Ultima LX series. Around $140 with post from the USA (some retailers here in Oz have them for more than $500). Only thing is they are REALLY BIG! I have the 8mm, and is one EP that is very easy on the eye - eye strain with EPs is one thing that's never mentioned!).
Then, the sky's the limit with $$$$.
Before you decide which EP to get, get to a star party or Astro club meet and try a few out. You can best judge what best works for you.
Mental.
Edit: Pippedat the post by Mal!
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