While money alone should clearly not be a deciding factor, I'd still point out that there's a very big financial difference between fostering and adoption. (I seem to remember that you're in the USA, Emily. I can only provide information based upon the UK situation but I'm sure that there will be similarities between the systems in our two countries).
Fostering involves looking after someone else's child, for which you get paid. Here in the UK, the payment is between £132 ($187) and £231 ($327) per week, depending upon the age of the child and the part of the country that you live in. There are no upfront costs involved in becoming a foster carer and the vetting process is relatively short (but still quite thorough).
Adoption means taking on a child as a member of your own family. The application process is lengthy (often taking around two years) and involves paying between £4000 ($5700) and £9000 ($12750) in court fees. Once the child becomes a member of your family, it's up to you to pay for their upkeep, so it can result in quite a big hit to your disposable income.
As I said at the start of my post, finance alone shouldn't be a deciding factor but I think that it definitely needs to be considered!