Even though it's my boyfriend's invitation, is it normal to feel bad not helping him with the vacation cost?

We're been together for 3 years and the last year has been stressful for him; ongoing issues with his two siblings and his job undergoing frequent changes. The one-week getaway has been scheduled from July 6-12 and this is the part that makes me feel bad. I've offered to help with the vacation costs but he doesn't want me to. From my understanding, if it's a vacation for two then generally it's 50/50; both contribute. He said thank you for the offer but do not help him at all, that he'll assume the cost for the trip.

Even though he clearly refused my help, I can't help it but feel bad for letting him pay for it all by himself.

Even though it's my boyfriend's invitation, is it normal to feel bad not helping him with the vacation cost?
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