Best time to negotiate and buy a BNIB spare playfield is when you order your BNIB game. Plus any spare parts or plastics, translites etc you want.
I always buy a signed Translite for every game and put in the signed one.

Dorothy for Stern USA who handles all those orders is a dream. You then co-ordinate her to speak with AMD after you have established stock availability. Been dealing with Dorothy for years and she is such a great person. Efficient friendly and professional.
Spare playfields are rarely needed in real life but a great insurance policy to have for resale.