Inexpensive sweet vermouth?
Carpano's out of the budget - what's a cheap alternative for mixing in gin cocktails?
Big fan of Lillet, which is similar aperitif from Bordeaux, and runs about $15 for 750 mL. Mix it with gin and vodka and you get a Vesper. I agree with Maximilien on Noilly Prat, which is also under $20 for 750 mL.
Curious why you are mixing Sweet vermouth with gin? It's generally paired with whiskeys.
Inexpensive? Martini & Rossi.