yurka--I think you answered your own question.
If you are in the middle of some program (like word, spread sheet or email processing) you will lose whatever data you have entered (including posting a reply here). It is the same as if you shut your PC off at the tower before you have been able to "Save" things. If you do not mind that, then I guess you do not need a UPS.
As you note you should have a surge protector to protect your computer equipment from being fried. However, I doubt a $5 surge protector is going to give you the protection you need--especially the power surge that might occur when the power goes back on. Calculate the volt-amps used by all your equipment and buy a surge protector accordingly.
Not all equipment need be connected to a UPS, but all should be connected to a surge protector. That includes your HI-FI gear, your cordless telephone, etc.
I checked out the cost of a new battery for my APC UPS recently. It was about 40% of the cost of new UPS.