It depends on what you have already. I started with Mr. Beer too. First, I got tired of using the PET bottles so I saved some old Sam Adams bottles, bought some caps and a capper. Then, I realized what SG and FG are and I had to have a hydrometer and sample tube. Then, I realized I needed a bigger pot so I bought a 5 gallon brewpot online for $35.00. Do you see where I'm going with this? I already had some things that come with the kits. When I was ready to go 5 gallon, I bought a Better Bottle, autosiphon, and racking cane. Poof! 5 Gallon batches! It didn't make sense for me to buy and entire starter kit. However, if all you have is a Mr. Beer fermenter and some PET bottles, maybe a cheap kit makes sense. It's cheaper overall then buying individually.
Or, piece together the way I did (even if it was inadvertently). Minimum for 5 gallon batches (as I see it):
Brew pot big enough for partial boils (say, 2 to 3 gallon boil minimum)
Fermenter (cheapest is 6.5 gallon food grade plastic bucket with lid)
Airlock (dirt cheap)
Bottling bucket (same as fermenter except with a spigot on bottom)
Bottles (not twist off's) (your Mr. Beer PET bottles work but you won't have enough for 5 gallons unless you bought more previously)
Caps (usually come with ingredient kits)
Auto Siphon (not required but highly recommended)
Note: on the cheap, you could even ferment in a bottling bucket and skip the separate fermentor to save money. Just buy a lid for it. However, you will have to bottle straight off the trub which I suppose you're used to anyway from the Mr. Beer fermenter's.
Good luck and welcome to the obsession!