A new transmission is expensive.
A fluid change will help keep the transmission going for many years. Poorly done transmission service can do more harm than good.
If you go "cheap" you may get more problems.
I don't like trans flushes, some are done right, many are done wrong.
A trans fluid drain, refill and a new filter is best. If the cheaper shop uses generic trans fluid that isn't good. Toyota brand fluid is your safest bet.
Don't shop so much on price. Shop for how the trans will be serviced? What brand of fluid will be used in refilling it? Definately avoid any chain oil change shop trans flush service, they are just not good. Get a price from a Toyota dealer. If the dealer price is reasonable that maybe your best choice to make sure it is done correctly.