I have tried explaining this to them but falls on deaf ears
They sound oblivious. I'd find that ghastly, but only you know their better points, & what their friendship is worth to you (in emotional terms, not financial!)
How committed are you to staying friends?
If you want to still keep seeing them on the events you can afford, you might find it useful to have a "broken record" approach to them so rudely dismissing your explanation of having short funds.
So you have a stock phrase you keep on repeat. Saves you from wondering what to say, or falling down the rabbithole of JADE.
(don't Justify, Argue, Defend,Explain).
Something like "no, I won;t be able to go on that trip, but I look forward to the next one".
& if they continue to demand reasons "no, it's not affordable now, but I look forward to the next one".
Any further pressing (& only a very dense or thick skinned person would do so) - you just revert to either of those 2 phrases.
Do not deviate from them.
Your friends will catch on eventually, when you stick to your guns.