We have two children aged 4 and 8, no childcare costs. CTC is £90 pw, with one parent working f/t on minimum wage. WTC is £40 p/w. We get £130 p/w.
Although they actually owe us money for once, normally they miscalculate and we end up owing them a fortune. This time they suddenly decided that DH was no longer entitled to the disability element of WTC. After arguing this point for a year and then giving up, they've now changed their minds and conceded that suffering from epilepsy does indeed constitute "Suffering seizures and/or loss of consciousness" and it is indeed a condition that would "Limit work opportunities" The two things they told us we didn't qualify on when they took it away 
Our payments will go up by another £40 or so p/w once they reinstate that and they owe us around £3k, although to be honest, I think DH would rather not have the seizures, loss of consciousness and limited work opportunities and give up the money.
I could believe that you might get somewhere near £600 p/w WTC and CTC if you had three children and applied now, but that's because it should have been spread out over the year, not Jan to April.
£649 p/w from now until April equals around £10k per year in Tax credits which seems about right to me.