Budget - two week trip to Norway, Sweden and Finland

I can not, sorry, but I have found Rick Steves' guides to other countries help me a lot in budgeting. He has a section on that in each book.

Without knowing the standard of accommodation you require, your mode of travel within Scandinavia, your dining habits, and where you are traveling from, there is really no way to even give you a ballpark estimate.
That said, restaurants are quite expensive in Scandinavia generally speaking...a meal in a "normal" restaurant will run you about 30 - 50 euros per person depending on if you have alcohol or not.
There is a wide range of lodging that will start around $100 a night for low-end hotels and increasing from there.
Car rental is quite expensive especially if you plan to pick it up in one country and drop it off in another. Gas will be about $7 - 8 per gallon (US dollars). If you plan to use trains and/or buses the cost will entirely depend on the distance and route you choose. Flying between cities in Scandinavia will save you a lot of time and will be no more expensive than renting a car or taking a train. Cheaper in many instances.

Your big ticket cost in the Nordic countries will be transportation and lodging. You can estimate for yourself the cost of transportation and lodging through the power of the Internet. It just takes a little research work by trial booking your prefered modes of transportation and lodging.

Typical Nordic hotels (and hostels) include a substantial breakfast buffet with their lodging price or as an add on. Eat a hearty breakfast and do the Nordic light lunch. That just leaves dinner as the big budget variable. In Norway, alcohol can be more than half the cost of of your evening meal if you indulge.

The bigger cities (e.g. Oslo) have city travel cards that combine transportation and attraction entry fees. The cost of these passes will give you a rough budget of local attractions and local transportation.
https://www.visitoslo.com/en/activities-and-attractions/oslo-pass/prices/
https://www.stockholmpass.com/what-you-get/
https://www.helsinkicard.com/

And of course just walking about is free:
http://www.visitfinland.com/mystay/product/free-walking-tour-helsinki/468/
https://www.visitnorway.com/plan-your-trip/travel-tips-a-z/right-of-access/

I have taken 14 RS tours in the last 16 years (soon to be #15 & #16) and spent time in all of those countries either after or before, or both, of the tours. I found the Scandinavia countries to be the most expensive (after Switzerland) of any country we have visited.

Norway in particular was very expensive for everything and we seek out inexpensive ways of traveling. I rarely recommend a potential traveler to use a travel agent, but in your case that might be money well spent.