I dont think deidara costs that less. He should be around 15k to 18k, i suggest you to save 20k coupons and wait for Fuku rebate, maybe you will get him in that event for free. In anycase i suggest you to wait until u get 20k coupons and wait for coupons rebate, you can easy get 2 ninjas or even 3 worth 15-20k each :)

Can you tell me about that more specifically?

There is an event called "Fukurokumaru's Deals" where you spend your coupons/ingots and you'll get items/rare ninjas back based on how much have you spent. Like this week: f you spend 20k coupons you can get 80 frags of jonin minato and a lot of other good staff. I remember when Angel Konan was released, i spent 20k and i got 80 frags of edo deidara from Fukurokumaru's Deals rebate :) If you are a f2p or a low spender i suggest to save your coupons and wait a good opportunity to spend them!

He's in a good bit of events, but there are a few factors.

How many you get for free is one of them, in mission boxes, at the moment, you can get a total of 12.5% of him, or 10 fragments, for pretty much free, as long as you do all 10 missions daily. (And you'll be halfway to one extra fragment), so it's best to go after him pretty much whenever, that'll help lower down the final price.

His price varies through events, but Herz has covered that part, my only suggestion is to be sure to pick up whatever free frags you can.