James Roberts would be silly to turn down a $300k or so a season pay increase thinking it's either that or a premiership if he signs with Broncos.
At the end of the day no team or coach can guarantee a premiership. But that $300k in the bank is guaranteed. I also don't think his Origin hopes suffer too much whether he goes to Broncos, Souths or Dragons. If he plays fullback for Dragons that may even increase his chances. More likely to beat out Moylan and Tedesco at fullback then he is Dugan, Morris, Jennings, Fergo etc. For a centre spot. Especially when neither Moylan or Tedesco has experience to give them an edge.
Only way you knock back what the Dragons are offering is if there's some massive payments elsewhere making up the difference.