This is probably one of the most frequently asked questions and the hardest to answer. Some people spend very little on souvenirs and others like to shop. Some people love to buy trading pins and that can add up quick. Others stick to one small thing. I can tell you that I just got back from an 9 day/8 night trip and bought:
1. Mickey head silver necklace - $16.95 + tax
2. Disney tote bag - $24.95 + tax
3. Lanyard and 8 pin starter set (promotional deal) - $15.95 + tax
4. Dooney & Bourke Mickey Ballon barrel style handbag - $220 + tax
5. Set of 6 holiday antenna toppers - $19.95 + tax
6. Mickey wine stopper - $8.95 + tax
I will also say that I was traveling with $295 in free money from credit card points and $100 Disney gift card from my boss, or I probably never would have bought the Dooney handbag!
I also have to say that I have multiple trips to WDW under my belt and don't need to load up on a lot of souvenirs. I bought the tote bag to replace one I bought about 12 years ago and is now looking pretty tired after daily use and the necklace to replace one that I also bought years ago that recently broke.
If you're on a budget I would set a certain amount for souvinir shopping between the two of you. I typically wait until the end of the trip to do my shopping so I've had a chance to look at everything and be sure of my purchases.
Have a great time.