If you buy a MM, you better be ready to set aside 6~8 hours a day for hunting sessions, and have enough ped on hand to use it for that long, AND be able to hang through the low loot periods.
I can't speak from experience of owning or using a MM (I wish...!) but there are some things here that others aren't saying. Weapon used affects your loot. The guy above me saying use HL18/HL20 and see how your loot is, and just adjust decay? Won't work. What you get in loot with HL18/HL20 is NOT the same as what you would get in the loot with a MM, so you would get false statistics.
Good tactic with a weapon such as a MM is to hunt high-MU drop mobs, such as Fresco, Scip, Chomper (to name but a few) because their high regen is virtually negated by the MM. There are a ton of other mobs you can hunt effectively solo, or in a team (can plow easily through most of CP with a MM, provided you have good skills/fap for it).
But I warn you, this is not an investment to be taken lightly. You will have to invest greater than 1mil peds ($100,000+) if you are a "lowbie", in order to reach effective eco status on the gun, AND have the relative skills to match (evade, paramedic, health, etc). This isn't counting a "regular" purchase such as Adj Fap, or armor such as shadow/angel (although Martial SGA would probably be a good purchase at this point).
My suggestion is to talk with current owners of MM's (Skippie and Blackhawk come to mind first, as well as Stryker and Grave) and get their take and opinion on how well the MM performs for them, and if they 'profit' or not.
Sadly, a lot of people I speak with ingame who say "Ya, we profit all the time!", I get clarification - and their "profit" is their skillgains... which doesn't count until you chip them out!
Now on the flip side of this, is the range of UL weapons we have. Genesis Firefly UL, Niflheim UL, Emik X5 - a lot of these are way way cheaper to get and use (skills included) than the MM is, and get maxed far earlier than MM would (40~70 pro level vs 100 pro level) - and give you the option of chipping out the 'extra' skills above that level for extra profit. There are some that also are incredible damage dealers for decent eco (Niflheim comes to mind, and I'd assume the HL18 SGA rocks too
).
Either case, and this is putting it VERY bluntly - consider any money you put into EU for an upgrade such as this gone and burned.
You won't be withdrawing (outside of an ATH) anytime for probably at least the first 6-10 months. You will need a high ped base so that you can stockpile ALL your loots, and sell at market for maximum return (101% beats TT for 1k ped+ of common crap) - and I'm talking probably 20k~40k ped that can just sit ingame and be cycled as ammo into loot, and won't slow you down for you to just sit it in storage until market hits peak on it, and then sell it for maximum profit.
Whatever your decision, make sure you do full research yourself into what you want to do... ask yourself questions such as,
What do I want to hunt?
What do I want to loot?
Do I want to gamble? (high regen mobs w/random uberhof chance, but low loot, vs regular mobs with constant loot)
Do I really have this much money to say "byebye" to for some years?
Final note - highest chance of continual profit in this game is a Land Area (preferably several...), plain and simple. Problem is, you need a good land area with good DNA (unique is a nice addition) in a good location (well... now with ability to buy TP's for 25k ped, location isn't much of a problem) and it has to be managed well (events, advertisement, etc). And unfortunately those tend to be excess of 200k+ ped... closer to 300k+ - and all the good ones are currently taken
.