This was something that would have been a HUGE help to DH and I when we got into the process. The agency tries their best to tell you but we found many more things that we weren't expecting. Maybe it would be helpful if everyone could post things they had to pay for. I encourage others to add, subtract, edit in any way. I won't put a dollar figure because it really can vary. In the end for us however, we found many things we hadn't taken into account that we would need to pay for.
Homestudy
INS fees
Agency Fee
International Fee
Mailings
Certifications
Authentications (this one REALLY added up!)
Reports
Education
Hotel/apartment for two trips
Meals in country
Airfare (two trips, one way return ticket 2nd trip for child)
Passport (if you don't have one)
Passport for child (if not included in agency fee)
Visas (for two trips...cost depends on length of notice to travel)
Medical exams that are often not covered by insurance
Medical exam of child beyond Embassy requirements (ie: the Embassy required us to have additional blood work for our little one before giving her a visa)
Gifts
Post placement exams/reports (may be included in agency fee)
Travel insurance (personal choice)
I haven't included the non-paid hours upon hours of your personal time, some which may require time off from work or vacation time needed to travel. I think that part is just getting you ready for what's about to come home with you.
I hope this is helpful and that others will chime in.