I would say no for several reasons.
First, I'd feel obligated to repay the coworker, which would then eat into next week/month's budget. No bueno (especially because who knows where lunch would be and what the co-worker would want to order; it could end up accidentally coming out unfairly balanced). I also think it's a good lesson for others to occasionally hear, "No, it's not in my budget," which is different from, "I don't have the money for it" because it goes to show that not everyone needs to be instantly gratified, y'know?
Was so nice of the co-worker to offer though! I'd just say, "Thanks, not this time, but see you next week!"