Yes, it is cheaper and best used in removing phosphate in bulk. It can scrub phosphate down to a lower range but you will usually need GFO to get the lower end. It is potentially toxic to fish which is why it's application in a separate tank like yours is ideal for treatment.While you can directly apply it to the curing tank and remove the participate it creates with water changes, I feel it best to drip in a 10 micron or smaller filter sock to catch the precipitate ensuring none make it to your reef tank.
By the time you order lanthanum chloride and the filter sock though, it may start becoming cheaper to just run GFO if you have it, especially if this is a one-time application of it as it is dispensed by drops and you'll have a whole bottle of it leftover.
If you just want to run lathanum chloride without the sock and just rinse your rock really well afterwards, you can order the lanthanum chloride online. I bought the commercial version but I think there's a cheaper version.
https://www.amazon.com/gp/aw/d/B00CMQZZJ6/ref=sxts1?ie=UTF8&qid=1480863191&sr=1&pi=AC_SX236_SY340_FMwebp_QL65
Also, some have been using "phosfree" from local pool supply stores. I haven't used that so I would research just in case.