That is a very pricey for those lights. If you want to go with those, check out an electrical wholesaler and just pay cash for them.
Those are an oddball light and used to be used in valence lighting, and because they are oddball, there are not the extensive production runs as there are with the four foot tubes, which tends to drive up the prices for the oddball bulbs.
T12 tubes are not a very efficient light either, with T8 and T5 fluorescent tubes being better options.
'K 'stands for Kelvin, and it is supposed to be representative of a theoretical metal being heated to those Kelvin degree temps, and with the higher the Kelvin temperatures, more blue light should appear in the lighting spectrum.
2700K is a warm white, 4100K is a cool white, 5000K is sometimes called full spectrum (it isn't) and 6500K is called daylight (1:00 pm outside).
dvg