48.6875
In the long run, the Sherpa seems nicer. For those that want to see mathematical proof:
With a metal silver sharpie: $7 + (x)(y), or $7 + (3)(x) where x is the amount of sharpies you'll ever use (substitute $3 for y).
With a sherpa: $30 + (x)(y), where x is the amount of sharpies you'll ever use and y's the cost of each sharpie (although this formula doesn't account for how you're buying the sharpies - I've seen 1/$1 and 12/$8.49)
Although my math may be flawed on this, the sherpa will save you more in the long run. That, and it's a lot easier to find sharpies than it is to find the silver sharpie refills around here.
I tried these formulas to account for 25 sharpies, and here's the simple math. With a sherpa, it costs ~$48.69. With the silver sharpies that require refills, it's $82.
Although I don't know how many refills come in the pack for $3, but it seems like it's 1. I don't see why anyone would pay $3 for that...