lowers DS sentrygun to 200 from 500 (#5225)

# About the pull request
lowers price of DS installed sentrygun to 200 from 500.

# Explain why it's good for the game

Based on git lense, the cost of 500 points has been fixed for at least 6
years ( from what I understand form the moment the fabricator was
added?). The sentryguns are not worth 500 points at the slightest, from
dozens of rounds of experience I can say that the fire off 20 shots max
before they die ( unless you do some DS hold after hijack, but that
hardly counts). The main issue anyone has with it is when you install 3
of them in south part of DS and I am willing t take suggestions how to
adress this issue, but it is minor one, as xenos never realy have to
fight them as they basicly get to shoot only while marines are pushed
into DS and are about to evac and queen can hijack without getting
anywhere near them ( or can just let marines evac and then take out the
sentryguns with ease after she calls the ship back down). More marines
evacing thanks to sentrguns might make the hijack more enjoyable for
both sides. Also lowering the price will only mean you see sentryguns
installed on ds more often, buying 20 of them will have no bigger effect
then 7.


# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

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screenshots and the `<details>` tags.

</details>


# Changelog
:cl:
balance: DS installed sentrygun price lowered to 200
/:cl:

Co-authored-by: Zonespace <[email protected]>

code/modules/cm_marines/dropship_equipment.dm

@@ -161,7 +161,7 @@
 	health = null
 	icon_state = "sentry_system"
 	is_interactable = TRUE
-	point_cost = 500
+	point_cost = 200
 	shorthand = "Sentry"
 	var/deployment_cooldown
 	var/obj/structure/machinery/defenses/sentry/premade/dropship/deployed_turret