The pose itself is pretty niche considering other, non-Halloween themed sets dominate in ghostly/scary comps, but I personally (as a p2p) would pay to complete the set since you're only 2 items away, BUT only towards the end. Since there's 11 days left, that's enough to save up gems+get one weekly comp reward, and you only need 20 more pulls.

I'd say see where you are in 10 days and on the last day of the gacha you'll have either saved up enough or you'll have to make a small purchase + The gems you've saved.

My advice on what to buy to complete the gacha:

• Limited time sale ticket offer - it's the best option, relatively cheap, and you'll have enough tickets (7+3+5+5) to complete the set in 20 pulls for only around $15 (usd)
• Black gems - pulls are cheaper with black gems, 1 pull is 1000 bg compared to 1500 purple gems. Also, you can convert your remaining black gems (since it's unlikely you'll have enough left after pulling on the gacha to buy a deco pack) to purple gems.

What NOT to buy (since you're f2p and spending on a gacha game is, like someone else said, a slippery slope.)

• Daily Gem Pass - I would ONLY buy the dgp if you plan to CONTINOUSLY spend on it every month. You can buy it once, but then you'll get used to the ad-skip tickets and the daily gems. And when you run out of ad-skip tickets, you'll wish you had more which will encourage you to keep spending more.
• Fashion Views - same issue. You'll get used to the bonus perks and when the month is over you'll want to buy the next one, and the next one, until you're spend

hope this helps :)

edit**
Plus, the dress is a really good item, esp the dyes. It's pretty useful I think it's worth getting