Assuming no benefits from the job, playing poker is better in this case.
20x30x4=2400/mo
9x40x4=1440/mo

You also have to add in a factor to correct for some things in poker, variance being the greatest. Plus your taxes will suck a little more (you will probably pay more). The games might get harder, etc. So I multiply the 2400 by 0.8 and get 1920.

Next you have to figure out your monthly budget, bills plus irregular expenses plus lifestyle costs. If this number is less than 1920 (which it should be if you're only making 1440 a month now). Then you have to decide if the difference is enough. For example, if you're total expenses are 1200 a month, your difference would be 720, and your bankroll would grow this much per month allowing you to take shots and have more cushion. If however, you're bills are 1900/mo, then your BR will grow by 20 a month and you will pretty much grind 25nl forever.

So I'd say x = 1500. If you spend less than that a month (including everything) then go for it.

disclaimer: needless to say i'm oversimplifying a lot. you could find a better job, get sick of playing 30 hours a week plus the study it would take to stay sharp, etc.