Once you buy/make the extra equipment needed, then it becomes cheaper, because the most expensive material the grain, is cheaper than buying extract, extract is more expensive because you're paying a maltser to do what we who all grain brew do ourselves extract the sugar from the grain. Especially if you buy bulk. And with AG you don't need the ferrarri of equipment unless you want it.
You basically need to upgrade to full boils if you haven't already (unless you're doing 1 gallon or 2.5 gallon batches,) which would mean usually a turkey fryer (and many of us start with the 7.5 gallon kettle that comes with the fryer, though you could spen more for a larger kettle) you then need to upgrade how you cool a full boil if you don't already have some form of a chiller. Because it's much harder to rapidly cool 5 gallons that it is to cool 2.5 gallons. And you need some way to convert the grain to your own extract, which can be as simple as Brew in a bag, with a muslin bag, it can be done zapap which is two buckets nested within each other with holes in one of them, it can be done in some way that involves heating the grain in a pot or kettle (Either in the stove or over or in some form of direct fire type method) or a converted beverage cooler.
How much money you want to put into it is really up to you.....You can do all grain on the cheap or you can spend 5 grand on a system, or somewhere in between.
If you already have most everything from already brewing extract except for example a Turkey Fryer, Wort Chiller & Beverage cooler Mash tun, you will only need to spend 25-30 for a turkey fryer, if you check the sales flyers or craigslist, and wort chiller if you don't have a cheap source for copper tubing will be 50-60, and And a 5 or 10 gallon cooler with either store bought modification or done with the countless plans on here, 50-60 as well. So for about 150.00 on top of all your regular brewing gear you can brew allgrain.
I helped a guy put together an ag system who never brewed before (so he needed everything from tubing to fermenters, a hydromter, etch) for EXACTLY 500.00 buying everything stock from the homebrewshop and hd for the turkey fryer. That was without making anything or shopping around or buying used, he had a 500 gift card from his FIL for Christmas, and in a day we got him up and running.
I buy bulk grain through a buy we do on here, 35 bucks for a 55 pound sack of two row. If I use 10 pounds of base malt per batch of beer that's 10 batches of beer for 35 in just base malt. I remember paying anywhere from 35-50 bucks for one extract recipe's worth of extract alone. So you can see that is a substantial savings. I still obviously need to pay for hops, and yeast, and unless I make my own specialty grains from the base malt, I still need those extra grains. But you can also buy hops and specialty grain in bulk to which further reduces your cost. And you can recycle/harvest your yeast as well.
So you can easily go from 50-60 dollars for an extract with grains kit for under 30 bucks for brewing an ag recipe....even cheaper. I think the cheapest AG batches I've brewed have cost me less than 10 bucks.