Red Leaf High

It's 10% per run, with nx bought key or daily key, and it scales so you will always get the same 10% every run regardless of your level.
Yes, I forgot it actually decreases, but it's only every 10 levels and it's only 1% exp decrease, so at lv199 you'd be getting like 6% exp per run, iirc.
Except it's not that great at lower levels (1-170/180) because until those levels training is just easier and less expensive.
It usually takes 10-15mins regardless of funds, because mobs you must kill there also scale with you.
Even if it's great exp it'd be just too expensive that early, so I recommend you train as much as possible before/during the event (obviously before it's over and having enough time to actually get 200) and when you're 180+ waste your money on those keys.
Try youtube for a vid on it, I am too lazy to try and remember every stage.