It sounds like you really want Silver, and are willing to spend some $$ to get there. If both are true, I'd recommend holding on to your $$ until after Jan 1, then waiting (all year if need be) until a REALLY good deal comes up. Realistically, you could get 13,000 miles by flying two transcon's with convoluted routing (i.e. EWR-MSP-HOU-SEA-HOU-MSP-EWR), but you'll have to wait until a deal comes up. I usually will jump on a $150 or less transcon if I can get the routing to work out. Add a few mileage runs to your normal (I assume) 12K per year, and you'll have spent about $300 extra on Silver, which may be worth it.

Also, look in to getting Silver via segments on CO. It takes 30 segments (one flight number = 1 segment)