Taxis in Sweden are deregulated. Thus, you don't pick the first cab in the line, as is the practice in most countries. The cabs are free to set their own prices, and some offer a fixed price to the airport. The advice is to pick one of the big firms mentioned above, but you could always haggle with one of the small operators. The norm is that most Swedes and tourists get ripped off because they are not aware and/or bad at haggling.
I wouldn't classify the Stockholm subway as being "very, very cheap". If you buy a single ticket for a short distance it's SEK 40. That's probably the highest in the world if you think about it.
