I haven't done this tour, but I have been to Scandinavia, so I can answer some of the questions.
"This tour is a bit more expensive than some of the other tours. Did you find that spending on your own time was also more expensive, i.e. food, transportation, admission fees?"
Yes! Scandinavia is expensive, and that why this tour costs more than some others; you will also see this as soon as you try to buy a newspaper, coffee, sandwich, etc. Rick's Scandinavia book has great money-saving tips. The one I remember is that many Stockholm restaurants have lunch specials; the meal is on the small side, but for about USD 10 you can't beat the value (for a place that might cost USD 40-50 at dinner).
You'll see that Sweden is more expensive than Denmark, and Norway is more expensive than Sweden. Norway is definitely the only country where some of my dinners were hot dogs from the local equivalent of 7-11 - for no other reason than to try to save some money! I didn't get to supermarkets in these places, so I don't know how those prices are. Supermarkets were a real budget-saver in Switzerland, another pricey country, and someone else posted that supermarkets in Scandinavia are not much pricier than in other parts of Europe.
Admission fees and transportation need not be expensive. I see your tour includes an Oslo Card, so those items are covered there. If you want or need taxis, those are very expensive, but the public transportation is excellent and not expensive at all. And any admissions you do have to pay are not high. Food and sundries are where the real sticker shock comes in (for instance, I'd hate to see what a tube of toothpaste or can of shaving cream costs in Oslo).
"Weather-wise, what is the best time of year to take this tour?"
Summer - long days and it never really gets hot, but is nicely warm and dry(er). But be aware that in this part of the world, that means June, July, and early August. By mid- or late-August, they're already transitioning to autumn (still nice, of course, but not really summer any more). And May is still spring - can be nice, can be rainy.
You'll see that this tour is offered only from mid-May through mid-September - that's not an accident. Any of these times can be fine, and of course there are no guarantees, but going in the middle of these dates (in other words, the tours leaving early June thorugh mid-July) would be my choice - particularly, as you said, at Scandinavian prices.