I have an external DLT4000 that ran me about $300 on ebay, plus I had to get a SCSI card. However, you can get an INTERNAL DLT4000 for like 50 bucks on ebay. If you are only going to write DLT's like once a month or something, put it internally. It takes up 2 CD ROM sized bays so I am assuming you don't have the room to leave it in there. Just pop it in when you have to make a DLT.
It's a great drive, takes the recommended Type IV tapes, and isn't expensive.