Three round types based on team economy: full-buy ($4,500+ per player), force-buy ($2,500-4,500), eco (<$2,500). Knowing when to pick each is half the IGL's job.
Eco rule: if your team can't afford a full-buy next round even after winning this one, eco this round to save. P250 + helmet only.
Force-buy rule: if you're behind on rounds and a force-buy could swing momentum, force. Galil/FAMAS + armor + a couple nades. You don't need to win — you need to win once.
Full-buy: AK/M4 + armor + helmet + utility (smoke + flash + molly + HE = ~$1,000). On CT-side add defuser ($400). Total ~$4,500-5,000.
Quick mental math: subtract $4,500 from your wallet. If positive, you can full-buy. If negative by $1,500+, eco. Between, force-buy.