So I it lately came to my mind, that I might be able to, kinda, improve some old left-over strings, the main problem of which is their greasiness, making them stiff and worsening tension. So would it be possible to dissolve the grease and oil by soaking the strings in alcohol or there wouldn’t be a big difference? 
I recently started changing strings faster than before, so was thinking about that…
Believe me it has been thought of like way too many times, the main problem is string breaking. Sure you can wash your string with soap and water and it will become cleaner, but it won’t improve strength. You risk breaking the string mid play and banging the yoyo around who knows what got hit, just to save up a little.
Someone also suggested heating the string a bit to melt some “fur” on the old string, I tried that, it’s possible, but not worth it.
If you really wanna save up, make your own strings, see my thread here Recipe: Simplest way to make strings. - General Yo-Yo - YoYoExpert that is the very basic way of making strings. You can use mini drill or something else to spin the string a lot faster.
The main problem is the fibers breaking and coming off. I have some string that you can wash and reuse, but it is quite expensive, at about 80-90 cents a string.
Of course, you are free to try washing the strings and tell us how it goes. I think you’ll find that the old strings feel very different though.
Some higher quality strings can be washed and reused, just throw them in your pants pocket before putting them in the wash and take them out before the dryer. It doesn’t work for all strings and bulk strings might not be worth the effort, but it doesn’t hurt to try.
All strings wear out eventually though, there’s really no avoiding it.
oh, well, I’m obviously gonna stack up on poly/nylon, and I’ll try cleaning some old ones, just for the lols they are actually far from breaking, don’t know why but at times the string becomes greasy really faster, way before it even starts to thin out. Thanks for the replies