The only real answer that I've found is "It depends"... on the plus side, it's a process, and it doesn't happen all at once. I've been reading on this a *lot*, so I'll break it down for you as best as I've been able to understand it. All guestimates are in $ US.
The standard order of steps, from what I've read hear and heard from others, for a successful transition are as follows:
Counseling (may be covered by insurance, may be free (community clinic), may be $60-150/hr)
Hormone Replacement Therapy (may be covered by insurance, probably $2-300 for endocrynologist and quarterly blood tests, between $30 and $120/month for meds)
Electrolysis (only for MtF, weekly or biweekly appointments for a year or two at $50-150 each, depending on provider... alternatively if you have a lot of time and not much money, you can apparently do it yourself)
You may be able (or have to) stop there, depending on your financial and/or physical/emotional needs.
For FtM, looks like top surgery is under $10k most places. MtF may not need/want top surgery, but implants are apparently less expensive (though not by much)
MtF may want facial reconstruction surgery. The sky's the limit on cost here, although many CIS women have masculine features, so this is not normally necessary for passing.
FtM: Bottom surgery- the sky is the limit here, too, depending on what is done, but at least hysterectomies are fairly standard procedures and well documented. Actual genital construction is where things get really pricey quick.
MtF: Bottom surgery- From what I've seen this costs anywhere from $13k to $30k for a reputable surgeon.\
The *good* news is that with health care reform in the US, some of these surgeries that are deemed medically necessary *may* be covered under insurance. Some other countries offer these for free, but often people choose to pay out of pocket for top-notch surgeons.
Hope this helps! There is a *lot* of information about this out there... remember, this is just the medical estimate... wardrobe, cosmetics, salon trips, etc are not included in these guestimates.