Cross-Sex Hormone Therapy isn't the same regime as Hormone Replacement Therapy. It's typically a cocktail of drugs carefully calibrated to enhance the visible characteristics of the target sex, reduce the visible characteristics of the original (birth, biological) sex, and mitigate undesirable or unexpected impacts. The effects of testosterone and estrogen are very different depending on whether they're applied to a body that has been producing them since puberty and now has depleted levels (e.g., at menopause) vs a body that has never produced them. And the level of knowledge about cross-sex hormone use is far lower than about same-sex; many treatements are still experimental and there is big money in research.
As part of a full transition, a hormone regime is carefully monitored and adjusted, initially at least month to month. The raw materials are typically quite inexpensive (could be as low as $30-50/month* for everything you need); it's the ongoing expertise and damage control that costs. There's a lot of money in insurance coverage as well, for those who can afford it. Unfortunately, some transitioners forgo a lot of this, especially in countries where many people don't have medical coverage or it doesn't cover this type of treatment or they're too young to access it, and use grey market drugs/DIY.
There's major money in cosmetic surgeries, and often the total cost there will include lengthy consultations, hospital stays, anaesthetics, medication, etc. in addition to the actual procedure. Your basic penile inversion currently runs around $140K plus another $30K+ if you want a facsimile clitoris. The standard surgery for women wanting to transition to male is only about $125K but does not result in sexually functional results. So again, more huge money is in research - a new method which MIGHT achieve functionality for FTM has been proposed but the price tag is something like $1 million per operation.
But these are the rarer surgeries, at least for MTF transitioners - supposedly in the UK, only 10% of the applicants for a GRC have had Sex Reassignment Surgery (SRS) of any kind. Breast reductions, mastectomies, and breast augmentations are a lot more common, as well as all kinds of cosmetic surgeries like facial feminisation/masculinisation, narrowing or widening of hands and feet, sculpting of various areas of the body (similar to cosmetic surgeries available to anyone, but in this case with a specific goal of superficial masculinisation or feminisation).
There's also, of course, a big market in selling a completely new wardrobe, accessories, personal care, cosmetics, etc. to someone who (1) is convinced that a new "appearance" is needed and (2) is insecure and sometimes even frightened about being identified as trans, or as the original rather than the target sex. Not throwing shade here, but crossdressers (whether or not they consider themselves "trans", although Stonewall UK says all crossdressers are) have historically been notoriously attractive customers for retailers because the element of secrecy meant one single customer might buy and dispose of a new outfit and "look" on a weekly basis or even more often. Hair is another big one, whether its removal of facial and body hair and often the need to cover up male-pattern baldness for MTF transitioners, or the encouragement of facial and body hair if FTM. You get the idea - anyway, it's not just estrogen!
*My numbers are USD, sorry - the current mid-market rate is 1 USD = 0.84 GBP if anyone cares to convert.