The big problem is that ironwood is temporary, 12 days at minimum level to create it. At 24th level and beyond, such full plate would be a magic item with a +1 enhancement while it lasted. The casting time would be 60 minutes (50 lbs. for armor and 10 lbs. for spikes)every caster level #of days.
The first time would also require a wood shape spell to shape the armor or an appropriate craftsman. The only comparable items on the armor table are wooden shields, so let's use them.
I would use the comparison between heavy wooden shields and heavy steel shields as a starting point. 7gp versus 20 gp. Make it a fraction 7/20 and multiply it by the cost of the spiked full plate, 1550 gp times 7/20 = 542 gp 5 sp. This is the cost of a suit of wooden spiked full plate made by either an armorer who specializes in wooden shields or a woodworker. What its weight is while wooden, I don't have a clue (heavy wooden shield versus heavy steel shield says 2/3 or 40 lbs.).
The spell still has to be cast on the suit every so often and if the druid is not high enough level then spellcasting costs (12th level caster minimum for 60 lbs.: 720 gp/12 days) will be assessed several times a month.
If you can't find a wooden shield specialist or a wood-crafter who could make the item, wood shape becomes necessary (with its 30% chance that moving parts don't work) though it is only a 2nd level druid spell and more easily cast than ironwood.
Of course, this is only my solution, not a rule per se.
Ciao
Dave