I tried this as a hobby a while back, and the initial outlay is indeed quite small, however, you are up against hundreds, if not thousands, of sites also promoting the same brands.
If you have little time at the PC, you will need the help of some good "cut & paste" web design software, and possibly a helpful friend.
Once set up, it will virtually run itself as many affiliate programs are now using dynamic banners (which means not having to constantly update the site yourself each time the casino does something new).
There is a small matter of reputation (unless you don't care), and making the wrong impression at the outset can be hard to overcome. Never promote a "bad" casino just because it offers a fantastic affiliate deal, you will get a negative reputation as a "bad" affiliate. Never resort to SPAM, again you will feel the heat, especially on here.
Have some kind of quality control, so that you can keep up to date on major issues that impact the brands you advertise.
I feel that personally written reviews can help differentiate your site from others, this is why Casinomeister, Simmo & Kasino King have had their success.
My own site was aimed at a small niche in the market, UK Fruit Machine players, who were getting fed up with crap games at 70% payout. It has not exactly been a success, and one reason I have been told is that frutie players trust online casinos even less than they trust the Fruit Machines!
I had ONE player, stayed for a couple of months, lost the monthly bonus and didn't seem to return. I made a magnificent 18
It did at least cover the registration cost for the domain, however it now sees a small loss.
I plan to revamp my site soon & have another go (now that I know more).
I need to get Frutie Players interested again, especially since two of the "competition" have ceased to exist