Current ballpark prices (I've been checking them often recently as I have 6/8 )
Dark and Light are around 200k
Ice and Thunder are around 50k
Fire and Wind were 150k when I got them a week ago, now 200k
Water is up to 400k
Earth is 600k
I have all but water and earth (I don't have an extra million gil laying around and when I get that million gil I'm buying Erase). I'll get them as I get the money, but I'm not going to drop everything to get them. They are important, but get the ones that will be most useful. If money is tight, buy 1 or 2 at a time as you get it. Start with Dark and Ice. Thunder should be next, then I'd say Wind and Fire, Water, Earth, Light.
I got my light staff for BRD, but shortly after pickup up BLM again. I don't cure much in xp parties, but I do find myself using it a ton outside of xp.
Macro your staves into every elemental spell you cast. Elemental debuffs, white magic debuffs, black magic debuffs, nukes, stun, aspir, drain, cures. If you don't know the elemental affiliation of a spell (which you
should, you're an elementalist) look it up!
http://ffxi.somepage.com/spelldb/class.php is a very good page to use.
Not everyone is filthy stinking rich; 200k isn't chump change for me. If you can easily afford all 8 staves, get them immediately. If you can't get them all at once, work on getting them over time but do get them.