The cost is really a minor issue compared to others, actually tape is probably more. I don't know what those discs cost but a tape is about $3 each if you buy in bulk.
When people hear tape they they think it must be an older technology which it is and the disc must be a better technology because it's newer but that is not the case. Remember DV is a digital format stored on tape and some people don't realize that, it's not the same as VHS or HI-8 analog recordings on tape. It's stored on tape as a bunch of 1's and 0's no different than a file on your computer. For example you can transfer the video stream from the tape to computer and you'll have an exact copy on the computer. You can transfer this back to the tape then back to the computer... then repeat this process 1 million times and you'll still have the same file you started out with at the beginning.
The reason tape is better is the DV format itself, it requires a lot of storage area that only tape can provide in such a cheap,small and portable package. You can fit about 14 Gigabytes of data on single tape in SP*
mode which translates to 1 hour of footage. A full size DVD can only store about 4 GB's or less. Because of this DV uses a much a higher bitrate than MPEG2, roughly 4 times more. With DV each frame is stored individually and compressed slightly. MPEG uses a compression method that utilizes what is called a Group of Pictures (GOP). Only 1 in 15 frames is stored individually, the rest is made up of information in preceding frames. This is why if you watch a poorly encoded video you'll see a lot of artifacts during motion.
MPEG presents some unique problems, it was intended to be a playback format and never really intended to be edited. If you want to make a simple analogy you can think of DV as being a negative and MPEG as being the picture itself. It can be edited but generally speaking most consumers are not going to have the software to do it properly and even if they do will not be utilizing it correctly and further degrade this already stepped on video. DV on the other hand was specifically made to be edited, it's even used in the professional world extensively and is supported by wide array of software.
Another thing to consider is reliability, DV has been around for many years and has already proved that it's a very reliable storage method. DVD discs on the other hand have yet to prove this. Keep in mind commercial discs are pressed, it's so similar to record that it's weird. Burnable DVD media is a technology that was brought to the consumer as an after thought. It was inveted to fit the disc, when you burn a disc that is actually what its doing. You're burning the data into a chemical inside the disc. This chemical will inevitably break down in the future. There's reports of cheap discs already failing, it is not a suitable long term storage medium. Just to add even new discs have problems, if you buy the best media on the market chances are 1 or 2 discs in 1000 are going to produce a coaster. The cheapest media may have a 50% failure rate.
More here on the specifics od DVD media: http://www.digitalfaq.com/media/index.htm
As I already mentioned the benefit of having a DVD camcorder is the ability to take the disc and play it in a DVD player immediately, just be aware that you are giving up a whole lot for this convenience. Also note that consumer camcorders that record to an internal harddrive also utilize MPEG@ so they fall into the same boat.
: While on the topic SP is the only mode that should be used when recording to DV. LP or other modes that will fit longer playing times onto one tape will produce the same exact quality as SP mode so that is not an issue. The problem is to achieve this the cam slows the tape down so it's writing more data in the same area. This lowers error tolerance levels. Tapes recorded in LP mode may be unplayable even right after you recorded them and most definitely will present problems down the road. If the specific cam they were recorded on becomes unusable you may find that you can no longer view them on other playback devices. If you value the material you are recording stick with SP.
If you have tapes recorded in LP mode I'd strongly suggest transferring them to your computer and back them up on a external harddrive because you're playing with fire leaving them on tape recorded in LP.