Short answer: No.
On the narrow issue: If a seller says that their prices are secret, I think you have to respect that. I'd contact them and ask if they really don't want price comparison engines like yours to include them, or if the "no trespassing" sign is for technical reasons. If the latter, perhaps they'll provide you with an alternative. If the former, then I'd say too bad, they don't get included, they lose some business, and it's their problem.
Tangential rant: Personally, I get pretty annoyed with companies that make me jump through hoops to find out the price of their products, places that make me call and talk to a salesman so he can give me a hard-sell pitch, or worse, make me give them my phone number so their salesman can call and harass me. I figure that if they're afraid to tell me the price, it probably means that it's too high.
In general: A robots.txt file is like a "No Trespassing" sign. It's the owner's right to say who is allowed on their property. If you think their reasons are dumb, you can politely suggest they take the sign down. But you don't have the right to disregard their wishes. If someone puts a No Trespassing sign on his yard, and I say, "Hey, I just want to take a quick short cut, what's the big deal?" -- Maybe I'm stepping on his prized Bulgarian violet bulbs and destroying a valuable investment. Maybe I'm crossing his people's sacred burial ground and offending their religious sensibilities. Or maybe he's just an ornery jerk. But it's still his property and his right. Oh, and if I fall into the dangerous sinkhole after ignoring the No Trespassing sign, who's to blame? (In America, I could probably still sue him for all he's worth despite the fact that he warned me, but is that right?)
