We are going the intl route (Kazakhstan) and during our research saw that the timing for payments varied by agency. We pretty much ignored (stopped looking into) the agencies that wanted large sums of money before they would answer questions, or wanted lots of money at signing. We just plain werent comfortable with that (personal choice) and eliminated countries and agencies that didn't fit that criteria.
For our agency, we paid our application fee ($200) up front, then our agency fee and refundable post placement report hold fee when we submitted our dossier ($2000 +$2000). That is all we have paid so far (not counting our homestudy, but that is from a separate agency). Basically, we are paying as we go. When we clear and are about to travel, we will pay the other fees (child placement and intl fee). Then we take with us our incountry fees.
Several people have posted from one agency (or umbrella type agency) that have paid their entire fee up front at signing (~25-30,000)!! These people are VERY stressed and haven't gotten to travel and have been in the process as long as we have (just over a year), and I can't imagine the extra stress that causes!!! Upfront or paying as you go, its up to you and you can pick an agency that has the same attitude as you do, they are out there!!
