how easy is it to make gametime instead of paying?
Hard to say. 20 hours of gametime is 280RTW points. The exchange rate fluctuates, but it's currently about $200 (in-game $$$s, not USD) for 1 RTW point, so you'd need to make about $2,800 for every hour of gametime in order to be able to keep paying for it. Considering I made $18,000 in my first 4-5 hours of gametime, that seems easily achievable (and I'm not even that good, I lose more than half my matches).
Of course, you'd have less money left over for gear, but TBH in 5 hours I haven't gone up in rank far enough to be allowed to buy any of the more powerful weapons, so first impressions is this isn't a game where you'll spend a lot of time farming money to guy gear with - by the time you're allowed to buy them, you'll have gathered enough money.
The above is assuming you're buying 20-hour chunks, obviously if you buy a full month you'll be able to spend more time playing and therefore earn a lot more in-game money.
Also, this is JUST money earned by playing and doing missions, I don't count money you can make on the marketplace. I spent a good 10 or so hours crafting stuff and selling it, and that's netted me about 100RTW points so far! Not even anything fancy, just minor stuff that I crafted because I wanted to use it myself, and then I made a few copies to sell just for the heck of it - a suede jacket, a paisley shirt (I know, I look like a pimp from the 80s

) and the Pacman theme tune (which now plays through the speakers of everyone I kill

). I imagine if someone's creative and can make nice stuff that people will want to buy they'll NEVER need to pay for gametime (not that I'm that creative or dedicated myself, I just see that it's clearly not just possible but easy).