This is a question from my 14 year old niece who is going to Paris in July with her parents. She is hoping to buy a small painting from one of the street artists and wants to know who much to save up for this purchase.

I realize prices of art vary widely, but do you think she might be able to purchase a small acrylic or pastel Paris scene (unframed even) for less than 100 Euros?

Yes!

she should be able to a very decent painting for about 20-40 euros for a pretty decent painting depending on the size. tell her to negotiate the price. you can always get a better deal than the first price quoted maybe even buy 2 for two thirds of the price. none of them are framed. she will have a blast.

Is this for 'street artists' in Place du Tertre, or are there other / better spots to shop?

It's on the place du Tertre that you will find the most painters. You could find some too along the Seine and on the bridges crossing it in the center of Paris, in the same area the "bouquinistes" are. But there aren't many, there.

Make sure that what she gets is really an original. I bought a watercolor from a young woman (she had many) about 2 years ago. I liked it right away, and noticed that she had about 3-4 prints that were different from the others. About 1 year later I discovered that "my original" was merely a repro. I still like it, but wish that I hadn't been deceived.

In addition to the Place du Tertre (where the artists can be unpleasantly aggressive), there are artists around the Place Beaubourg outside the Centre Pompidou. My son bought a nice original painting there two years ago for 20 euros.