The game is well worth $20. I didn't know you could even see how long you've been playing the game (must be hidden in stats somewhere), but when I played it non-stop for days, I went through at least 3 recharges with my DS, and I stopped playing once I beat the actual story, there's probably plenty of rampage missions and other bonus (read: waste of time) things to do that I don't bother with.
It's a bit on the easy side, but it's the most fun I've had on the DS possibly ever. It's not the highest rated game on the DS for nothing. It's a technical and gameplay winner through and through. If you've got prime or $5.02 filler already in your cart, I'd say buy it instead of waiting for the pricedrop. It's definitely worth it.