Hi Mathew, if you want Multiblitz branded tubes then there isn't a cheap solution, however, I doubt Multiblitz actually make their own tubes so you may be able to get another 'brand' to fit though of course what you're paying for with the Multiblitz one is guaranteed quality of the light output and of course the choice of warm or cold tone - £69 from MrCad. I've only had one tube go on my Multiblitz heads in 30 years and TBH it was on a 30 year old head so the replacement tube was worth more than the head, so the head was scrapped. I also have an old 900ws head the cost of the tube for that one if it goes will be £219 so again worth more than the head really.
Paul