For each game I assign players a percentage of their teams goals in order to price anytime and first goalscorer markets. I currently have Rooney in for 30% of United goals in games he plays and Aguero in for about 26% of Citys goals. vPersie I put in for about 33% of Arsenals goals. So far Rooney has 9/7, Aguero 8/7 and vPersie 2/7. At rate they are scoring at it is perfectly reasonable to expect United to score another 65 goals with Rooney getting 18 of them assuming some injury giving him a total of 27. City could get about another 50 goals (ten higher than last year in total??) and Aguero would maybe get about 20% of them allowing for injury and resting giving him a total of 18ish and vPersie could get 30%ish of Arsenals remaining goals (assuming 50 in rest of games) giving him 17 in total so not sure I really agree with adnmdv's totals. Aguero is obviously the difficult one because he could score a lot more goals and so could his team but the presence of Dzeko and Balotelli (and to a lesser degree Silva and Toure) means the goals and games are always going to be shared around a lot more. Rooney tends to play a lot more and is, vitally, proven through a British winter as is vPersie. Problem with vPersie is Arsenal look to be on a downward spiral and he is injury prone.
I actually think at 2/1 Rooney is cracking each way value at a quarter the odds first four with paddy power. 15 Goals will get fourth place in almost half of seasons and he is certainly no 1/2 shot to get 15 goals) and the win portion seems fair at worst.
I've a couple of minor quibbles with these numbers.
You have United to score 65 more for a total of 89 goals. That seems reasonable to me.
However you have Rooney scoring 18 of them instead of 19.5 (by your methodology of 30% of 65 goals) which equates to (if we accept United scoring equally throughout the 90 mins) Rooney having 2492 more minutes this year (+ 540 mins already played) for a total of 3022 minutes. That figure is too high for me. None of the top strikers (barring Darren Bent) ever play this amount of Premiership minutes. Rooney has a career best of 2923 minutes and averages 2461 minutes over the last 5 seasons.
I actually don't disagree much with your projected total for Rooney as I have him at ~25 goals in any case.
Secondly you have City scoring 50 more goals and Arsenal scoring 50 more goals? Surely that can't be right? Probably more like City 60 goals and Arsenal 45.
And you have RVP getting 30% instead of 33% (to accomodate injury/rest) of those 50 goals. That equates to RVP playing 91% of minutes in Arsenals remaining matches or 2536 mins (+ 606 mins already played) for 3142 total minutes. With a 5 year best of 2199 mins, that just doesn't look likely. In fact, it would be extraordinary. So I feel you're way overestimating RVP.
I totally agree on Aguero being the difficult one though.
I don't really know how to rate him, not having enough comparable historical data to base it on. Plus whoever is brought in to replace Tevez in the January window is likely to be top class and is therefore likely to be cup tied in the Champions League and is therefore more likely to steal minutes from Aguero in the Prem. Of course if City go out of Champs League, which is quite possible, then Aguero then becomes more viable again.
He has something like a 1 goal in every 2 games average over the last 3 years in Spain and doesn't seem to have had injuries in his last 5 years with 30+ league appearances in all of those years. Obviously he's already got off to a flyer in England and he seems to be the 1st choice striker at City. If his minutes hold up and Citys scoring holds up, I'd expect him to challenging Rooney all the way. But there are more question marks over him, no doubt.