In our case, we have a family plot of 16 graves (actually 8 that are double deep), purchased back in the 1800's. At the cemetary we use, you can bury ashes IN WITH an already filled grave. They are buried on top of the current occupants. My aunt opted for cremation, and to have her ashes buried on top of her father's grave (due to a mixup, he was buried in the top space, but no one is under him, so it's the same as a filled grave, if you're following this?). Anyway, I can't tell you the exact break-down of the charges, but we spent just slightly over $5000 for: the cremation, the opening of the grave, the concrete vault that was required, and the limos to take the family to the grave site for the burial. Headstone--well, I bought 2 of them, for her plus 5 other graves that were filled but not marked--and the cost was about $2200. So I'm estimating my aunt's would have been about $1000 for her alone if I'd gone that route.
Just out of curiosity, I did ask what plots in our 'section' were going for today--about $2200 each. That $100 my great grandfather spent on all 16 seems like a pretty good deal, now!
May I make a suggestion? If she wants to save a few $$$? She could hang onto the ashes, and let her next of kin know that she wants them buried with her, when the time comes. My plans are that when either my husband or I die, the other will hold the ashes until their demise and our ashes will be interred in one vault (assuming the cemetary will cooperate on this) in the grave that holds my parents. DH is on board with this--his family doesn't have a single place, they're scattered among several cemetaries. That solution will mean only one grave opening, and hopefully one vault, then our stone will be for both of us, made at one time--much cheaper than buying one, and later adding another name.
Unfortunately, it really seems a shame to buy a cemetary plot (they're SO expensive) just to bury ashes. However, if the cemetary is willing and will continue to bury others' ashes in the same plot down the road, it might not be too bad in the longrun.