Hotels in Helsinki are quite expensive by most standards, but at least the quality is usually very high so you may not feel like you are being taken advantage of. It’s tough to find a real budget hotel, though if you are coming on a weekend you might find prices at the nicer places are within your range since so many business travelers aren’t in town. Hostels are all fairly expensive by international standards as well.
Hotels in Helsinki are quite expensive in general, and especially during the busy summer season. This is a big business city so hotel prices are often cheaper on weekends, at least at the higher end places. From October through April prices are usually down a bit, and some hotels become real bargains if you can put up with the weather and short days. Hostels in Helsinki tend to be expensive as well, with little price variation through the year.