I'm kind of taken aback by some of the harsh replies.
It's always God's desire for a family to stay together if at all possible. Infidelity can be a just cause for divorce, but if the couple want to reconcile and heal their marriage, I think it's downright reckless to advise them to split up anyway. We don't know this man or his wife, and we only know a very basic story of the situation. To say she's "playing" him and he should divorce her... sheesh, I wonder if people THINK about what they're saying here sometimes. You don't just throw out stuff like that, those decisions are lifelong and life changing, and definitely not something to do just because some strangers online told you to do it!
Sorry, that just really hit me as incredibly wrong.
To the OP, I would say if you and your wife are willing to get into counseling and continue with the pregnancy, then God bless you and go for it. There's a lot of junk to be sorted through here, not just a pregnancy by another man, but whatever led to your wife's cheating in the first place. You'll also be dealing with custody issues once the child is born, so start educating yourselves on the laws surrounding that ASAP. You may be looking at having this other man involved in your lives permanently if he's going to fight for custody rights to this child.
As for your financial worries, you need to contact your local social services office right away. Your wife is probably eligible for free prenatal care, and possibly free food benefits specifically designed for pregnant women living on low incomes. Once the child arrives, you may be eligible for Medicaid (free or low-cost healthcare for all of you), food stamps, etc. Just look into it, it never hurts to ask. I'm not a proponent of using the government for sustenance, but when you're in a bind like this, there's no shame in it. You need help, period. Ask for it.
God bless, please get yourselves into good, solid, Christian counseling ASAP, and best wishes to you.