You do it yourself, of course it is much cheaper. The price I am talking about is you do it yourself. If you pay someone to do it, they might charge you about 1 hour labour, it depends on how much the garage chage you for an hour labour.
From what you describe the problem, it sounds like not the dirty transmission oil problem. When did you change your transmission oil last time? If you didn't change it for long long time, maybe it is good idea to change it anyway. Since you have to change it every few years, it depends on what kind of car you are driving.
You might say, oh if the problem is not from the transmission oil, why I need to change it.
But I can tell you, change transmission oil might cost your about $60 but if you don't maintain your transmission, it could cost you over a $1000 repair.
That why I suggest you change your transmission oil first to see if it help.
If it slove the problem, you are lucky, you save money from paying someone to trouble shooting.
If not, you still change new transmission oil for you car, which is required to change every few years anyway. So this approach, you have win win situation, even you want to sell your car, you need to fix this problem, nobody going to buy your car if it does run smoothly.
Unless the mechanic can tell what is the problem with your transmission, otherwise for trouble shooting process, you better start to change parts/something from cheapest price/most economical way.
Check your car owner menu to see how many KM you need to change transmission oil. If you still don't know, call the dealer, you don't need to bring the car there, just ask them how many KM you can need to change transmission oil.