Hey, tickets for X are only £Y if you're interested?

I would start the conversation somewhere around the fact that you can show them “Wow, look how cheap these tickets are! Would you be interested in going with me?”

You don’t have to explicitly say “I pay for me and you pay for you,” but it can be kept organic by sharing how cool you think it would be and asking if they would be interested in going at the same time. If they straight up assume you will be paying for them despite these social ques, then you can be more direct and bring up the idea “if you get your own ticket to this awesome concert, I’ll bring snacks or buy us a drink!”

This type of conversation can actually bring up cultural differences between ask/guest culture.

Ask culture is more straightforward--both parties say what they want. "Hey, I prefer going Dutch. What about you?" "No, I prefer if you pay for it." / "Yeah, totally fine." etc.

Guest culture is more like you gotta mind read.

As an ADHDer (and possibly autistic) I'm an asker in a guest culture myself. I used to get so stressed out about appearing rude. Now I'm like, "If a person finds me rude for asking something like that they're not worth being in a relationship with anyway."