I use GT-Shadow with RM settings.
I think that if you want to use 2000 usd account and not to add to it in the meantime, it makes sense to buy the lifetime. If you assumed even 6% monthly income, for a long time a subscription of the bot would eat away the profits considerably.
6% monthly profits are possible, but not with the canned settings that the authors provide. My initial account of 5700 has grown by 2600 in 4 months, but I used a bit more risky settings when AUDNZD was reaching the upper price bound. This month I am 240 USD up, but I also added to my account additional funds to make the trade size bigger. I scaled down AUDNZD a bit, so the growth will slow down. EURCHF provides about 20% of the trades, but it is very slow. The good thing is that it has 0
swap
rate 
, so it does not cost much to keep the trades open for a longer while.