If you received an inheritance, then I'd likely let it go if you have a strained relationship already. If it was coming directly out of your own pocket, then I'd likely speak up and tell them nicely that this is not something you're putting more than X amount of $.
If not strained, I'd just tell them that you'll chip in X and leave it at that. They can decide if they want to go ahead if they aren't getting it split evenly.
I'd personally just keep repeating "I don't think dad would have cared, or wanted us to waste his money doing this. A smaller stone isn't going to offend him and I think it would look nicer anyway. If you want to do this, then that's totally your call, but I won't be chipping in on that part because I feel like dad would have hated the fuss." And then just keep saying "I love you, but I am not going to do this because I don't agree with it."
Your dad is beyond caring and this is more for your siblings and needing to make things PERFECT as some sort of loving gesture (money = love when it comes to death unfortunately) that proves that your dad was important to them. The funeral industry knows this and they make bank off of gullible, vulnerable people driven by emotion instead of logic.
Consider the amounts spent as a from your dad's pocket in order to make your siblings happy. It's less coming to you, but you're beyond lucky to receive anything, and as long as they aren't being insanely stupid about it (like goldplated caskets and hiring mourners to cry at graveside every Wednesday for the next 20 years), deep breath, realize you're more logical and less driven by emotions and let them have this.