I always buy my ticket in such a hurry that can't tell the exact cost, but I believe it was around RD$140-160.
Near Metro in Santiago the only large place I remember now is Supermercados Nacional (my favorite). It's just a few blocks away from Metro.
I believe if you take Caribe Tours, you will be very close to Jumbo. Caribe Tours has two stops in Santiago. The first one is in Colinas. Colinas Mall is where Jumbo is (in addition to a regular large shopping mall). I never got off that stop though and don't know how many blocks away Colinas Mall is, but it can't be too far. (Colinas is a neighborhood). The second stop is one block away from Metro, in Jardines.
If you're planning to come back in Metro, the last bus leaves at 6:15pm. Before that one, you have one at 1:15pm. I don't think there any bus leaving in between those two.
A cab in Santiago from the bus stop will run you around RD$150 pretty much every where you may want to go. On your way back, you'll pay a bit less than that.