I've done this before; I bought a pair of jeans at 70% off that were two sizes too big for me and had them taken in. My tailor is famously cheap but EXTREMELY good, and it cost me $60 to get it taken in at the waist, hip, thigh, knee and hemming.
HOWEVER, let me note that the jeans just did not look the same. See the thing is, when jean manufacturers set up measurements for sizes, they also scale everything up or down depending on the size. So even though the jeans fit me, they still looked too big, and the bootcut ended up looking more like a flare on my small frame. Plus they simply were not as flattering as a pair that fit me right to begin with. And if you wanted to take it in at the ankle as well, you're basically asking your tailor to make you a new pair of jeans!
My suggestion is don't do it. Ultimately it's just not worth it. If they're old jeans that you love, pass them on and splurge on something new to celebrate losing the weight. If they're a pair you bought because you loved the look but they don't fit, take them back and keep looking. It really isn't worth it in my opinion.