I’m looking for string that lasts long isn’t expensive (under 6 bucks for 25 pack)
and is good for whips and slack and all around yoyoing but mostly for slack.
Thanks!
Edit:
so what if I raise my amount up to 8 bucks. Then what?
Not asking for much, eh? 
Kitty Fat would work.
I think it’s worth considering more expensive strings that last a LONG time. Some of the “boutique” guys around here make amazing string that just doesn’t seem to die. Quick 13’s T13x is just ridiculously long lasting. Toxic Dragon. Anything in that “slick trilobal” category, really. So you might pay $8 for 10 (instead of $6 for 25) but if they last days or even weeks instead of “a few hours”, it’s ultimately more economical.
Bought some T13x from Quick 13 like… a month and a half ago…? And I’m still on the first one. It’s not on a yoyo that I play every single day, but I’ve played it a lot.
Now, I’m not saying that’s the only option (to be honest, I like their less durable P1 better overall), just saying that you can’t look at “cost per unit” if you want to save money. The really low “cost per unit” strings tend to get cruddy pretty quickly.
Go get yourself some JAMS strings! Last a very long time, Ive been using the same string for weeks! Linkto their facebook page in my sig. Its $6 for 25 pack 
While maybe not perfect, but if we’re talking $6 for a 25-pack, that equates to $24 for a 100-count at full price.
That gives you YYE 100% poly and Kitty String for sure. However, neither last very long.
Then again, I’m spending $40 for a bundle of 100 Type X strings. They are proving their value to me every single day and meet all your needs.
Long lasting string is overrated. I get knots i either can’t or don’t feel like getting out long before i need to replace my string anyway, or the string gets dirty and dull. Kitty fat, Kitty nylon, Kitty 1.5(haven’t played, hear it’s good, buying some ASAP) are all excellent, inexpensive choices.
One of those YMMV things, I guess! I’m still too rookie to have gotten any knots that I couldn’t get out. Standard poly does seem like it would hold onto a knot better than slick strings, though… 
Dirt must happen, I guess. I notice it far more quickly on standard poly than on something trilobal-based. Not saying anybody has dirty hands, per se, but my hands are generally nutty clean because I’m always washing infant and young child off of me.
Whippy and cheap do not go hand in hand.
Seriously whippy strings are all in the $6 - $8 for 10 range.
The whippiest cheap string I could recommend is Fat Kitty.