My preference is for 15" ralley wheels. Only aftermarket special build wheels are 15" SBP and they are expensive. The question is based on the premise that with the adaptors I could leave my car SBP and use 15" Ralley wheels.
Rat
$110 for a brand new wheel is expensive? Where do you think you're saving money? A GOOD set of wheel adaptors will run at LEAST $100, and you're adding the expense of special offset 15" big bolt Rallyes (not stock), plus the questionable safety risk of using adaptors (even good ones) in the first place. I could see using adapters if you had no other choice, but why add the risk and complication of adapters for a set of wheels that ARE available in the size and bolt pattern you want? $110 a wheel seems like a reasonable price to me, considering aftermarket wheels can run over $500 each.
If you really want them, just wait until you can save up for 15" SBP Rallyes, and be glad they're actually available. SBP wheel options are very limited, and if you've found a style you like, you're lucky. Most people who switch to the BBP are forced to because they can't find a SBP style they want (i.e., they don't want Rallyes).
Wheels and suspension are not a good place to cheap out or cobble something together just to save a buck.