Cost is always important. This does not include the cost of travel and time. This is a considerable investment, but the cost pales when compared with the exorbitant expense of not training employees. Skimping on training can end up costing more in security, loss of data and increased downtime. The best value for basic training is usually community college or college extension classes, especially for common products like word processors and spreadsheets.
Some companies also guarantee their courses.
Interview instructors so that you can judge whether or not they will be appropriate and effective. There are some people who are very skilled technically but painfully boring. Others may be personally interesting and outgoing, but have little knowledge on the technical side. Professional certification helps but is not vital. Independently developed courses can be very good as well, but interviewing the instructor and checking references are especially important for these programs.