Sweden isn't cheap; people are well paid, which means costs tend to be higher than in most of the US. That applies to hotels, restaurants and transportation. Sightseeing costs are fairly typical for a capital city, I think. Some places have separate winter (lower) and summer (higher) entry fees.
Unless you're heading to an intensely popular summer destination like Visby, Stockholm is likely to be your most expensive destination, so I'd focus on it. Go to booking.com and take a look at hotel rates for that city for your dates in areas like Gamla Stan and Ostermalm. (There are less expensive areas.) If you see a chain hotel you like, you may be able to save 5% or so by joining its affinity club, and reserving through a hotel's own website is fairly often somewhat less expensive, but booking.com is an easy place to get an idea of costs. There may be substantial differences between midweek and weekend rates.
For sightseeing, go to the websites of the attractions you plan to visit and see what they charge. For example, the excellent and very popular Vasa Museum is Kr 190 in the summer. Skansen, the open-air museum, is Kr 245. The Nationalmuseum (art and decorative art) was free last year but is now charging Kr 150, or just Kr 100 90 minutes before closing (not enough time to see the entire museum). There are currently about 10.6 krona to the US dollar, so you can divide those numbers by 10 to get an idea of the cost in dollars at the moment. For the first two museums, you'd almost certainly be using city transit. which is Kr 38 each way. (If you're a heavy user of trams, buses and boats, a transit pass would probably save you money.)
I opt out of hotel breakfasts wherever possible to save money, but many Scandinavian hotels do not give you that choice. The hotel breakfasts are varied and generous. I imagine most tourists eat a light lunch after a heavy breakfast, so lunch needn't be expensive. There was a carry-out salad spot near my hotel on the edge of high-cost Gamla Stan where I could get a hearty salad, roll and soda (package deal, else I would have skipped the soda) for around Kr 125.
In the more touristy areas of Stockholm, I think you'll probably not spend much less than Kr 400 per person for dinner in an actual sit-down restaurant with table service--and that doesn't include alcohol--unless you go somewhere like a pizzeria or casual Middle Eastern spot. There are definitely opportunities to spend less; you can buy food at a supermarket (I ate a lot of cheese and Wasa crispbread in both Norway and Sweden), but will you? On a short trip, most folks want to take the opportunity to try local dishes rather than the sort of thing they could get at home (and for less money).