There are plenty of used drives on eBay for cheap, although burners are some of the least reliable computer parts so this can be a little iffy.
Considering how even the fastest DVD burners at 24x (33.24MB/s) fit comfortably within the speeds of USB 2.0, and how many modern computers have no optical drive at all, I would suggest an external USB DVD burner as it's much more likely to continue being useful to you in the future.