I'm with haps on this, at least to a point anyway. I personally never kept any of the supposed "starter" lizards. It's funny really because lately I'm interested in those and considering trying beardies or leos.
So anyway yes research. Just to give you a ballpark idea of what you're looking at (depending on what type of monitor you'd like to keep) here is a bit of a cost breakdown, from memory.
I built my monitor enclosures out of plywood with framing, glued and tacked. I had tools already so I only needed material, most expensive part was the plexi for the doors. Around $200 for 2 8' cages built bunkbed style. Not bad, but if you need to buy tools it'll cost you...and forget buying a cage, there's not a lot out there and for the price of some of the cage kits you'd be better off finding a local carpenter and paying him to do it for you. Then you've got heating, I used CHE for background and night heat and halogen bulbs for basking spots (great for creating hot spots and better on electricity). Substrate and cage furniture are the easiest, go find some logs and branches for climbing depending on what you get, and yard dirt for substrate. I've also used mulch, topsoil, and mixtures of both. Feeding costs can mount, I used to buy feeder rodents in 3 month supplies for my herps and as I recall I paid around $200 or so every 3 months (this was feeding 3 monitors, a retic, and a cal king so I'd order a mixed package). Shipping is the big cost there though so plan to pay near that for a 3 month supply of appropriate rodents; you could buy more to save on shipping but I don't like my feeders to sit in the freezer for long. Other costs will be similar to what you'd have with smaller herps (vet etc). Then there's maintainance...most of the monitors I've kept were surprisingly easy re: cage cleaning; they generally crapped in the same spot or did it in the water pan. Be prepared to do daily spot cleaning for the sake of both your monitor and your nose

I generally had to leave myself an hour out of each day spot cleaning up after 3 monitors, thawing their food and feeding, and dealing with other things that came up...but I think that's optimistic; if I had a large monitor who happened to be aggressive then feeding and cage maintainance would be much more time consuming.
Sorry for the long rambling post but it's really important IMO to have a really good grasp of what to expect when taking on any large herp, be it monitor, tegu, iguana, large boid, sulcata, whatever. Getting some hands-on experience with a large monitor would be a great idea, check out local herp societies, see if you can find somebody with some big guys and check them out.