Basically when I moved here, 25 years ago, I hated it. The Aga was a big part of my desolation. I was stoooopid. I now don't ever want to live anywhere without one.
In your case, so many variables. The cost (and the performance to a certain extent) depends on not only the fuel, but what it is actually running in the house.
Fuel:
Oil - variable cost depending on fuel costs outwith your control - however you may be moving somewhere where the whole village is on oil and they buy en masse which brings the cost down.
Gas - the expense depends largely on what the Aga is doing. I did consider getting my Gas Aga converted a couple of years back but when I investigated the cost of an electric Aga, Gas was cheaper. It may be different now.....
Electric - depends a lot on which type of electric Aga it is. There's a whole range of types - I recommend joining a Facebook group like I Love My Aga, or similar and asking once you know what you're dealing with.
Usage:
An Aga that is just for cooking (and therefore switchoffable in summer - regardless of fue type) will be more manageable, cost wise, than others that are essential for:
Heating
Hot Water
And what you can do with them depends on what other fuel types your home has available. For example, my Gas Aga used to run the hot water in Winter (Immersion when needed in Summer). I got a new boiler and hot water tank installed that runs the hot water as and when needed so now my Gas Aga is much more efficient and cost effective.
Having said that, when it is on, it warms so much of the downstairs (and the rooms above it) that I use the GCH far less than I otherwise would. So, horses for courses.
You've got to live with it and work it out first to see if it's cost effective. But, honestly, I think it's made cooking easier. Difficult to F something up in an Aga - possibly apart from cake, and even that only takes a little bit of practice to adjust to.
Should have added - no more kettle, toaster or tumble dryer in Winter when the Aga is on. I do switch it off over Summer as here in the SE it would cook us too if I left it on. I have a separate electric oven for Summer, and a microwave, and they've lasted years as they're not used that often.