I think I know. I compared it to the one calculated by PT3 (which shows both dealt and paid rake), and it seems that Elephant's rake is within margin of error of what PT3 says to be paid rake.
Now, to calculate which is better - the rake or cash bonuses, I have made the following estimations for the NL50 level I currently play (and hence have 30k+ hands to make accurate enough assumptions):
1. Average rake per raked hand = $0.051 (actually, I play on Stars, but I don't think this should be very different for Party)
2. Party gives you 10 party points for every 67 raked hands played at $0.25/$0.50
3. So, different bonus cash package levels, expressed in terms of rakeback would be:
Silver. $50 + $100 for 1500PP = 10050 raked hands = ~$500 of dealt rake. Equals 30% rakeback.
Gold. $500 + $1000 for 12000PP = 80400 RH = ~$4100 of dealt rake. Equals 36.6% rakeback.
Palladium. $1500 + $3000 for 30000PP = 201000 RH = ~$10250 of dealt rake. Equals 44% rakeback.
Palladium Elite. $15000 for 150000PP = 1005000 RH = ~51250 of dealt rake. Equals 29% rakeback. Except for the convenience of not having to clear the bonuses, I don't really get the point of this one... Perhaps I would if I were Palladium Elite though 
Of course, for rakeback you normally don't need to do any clearing or wait till you accumulate some amount of points, so it is more straightforward, but as you can see, cash bonuses can be more profitable.
Btw, PokerStars higher VIP levels are able to offer better cash returns, but that comes at a cost of lower level players:
SilverStar buying $50 cash bonus - equivalent of 10% rakeback
GoldStar buying $285 cash bonus - equivalent of 15.2% rakeback
PlatinumStar buying $650 cash bonus - equivalent of 21.6% rakeback
Supernova buying $1500 cash bonus - equivalent of 35% rakeback
Supernova buying $4000 cash bonus - equivalent of 37.3% rakeback
Supernova Elite buying $4000 cash bonus - equivalent of 53.3% rakeback
However, I guess Elite is hardly achievable on NL50 (Supernova should be possible though), so the last figure should be taken with a grain of salt...
