I think it would be a good idea to make a serious run at him. Gauge his interest and see how high his price tag is.
I would not want to pay him like a feature back, because I do not think he could be one and we already have one and his franchise tag number would be ludicrous as well.
But if his salary demands are reasonable, I think he would be a good fit. Especially since Jimmy Raye was Norv Turners OC in Oakland, so the offense should be familiar to Sproles if we want him to take a few snaps on offense as well.
One thing worries me, though: I just checked his stats on NFL.com and they are all down from last year, including his return stats. Was it just a down year for him, or should we worry about that? Because return men tend to lose their edge pretty fast, so maybe he already starts to decline? I am not sure...