Nothing improves a property faster per dollar than fresh mulch in cleanly edged beds. It holds moisture, suppresses weeds, moderates soil temperature, and makes everything planted in it look intentional. It also does real damage when it's applied the way most of it gets applied.
Depth first: two to three inches is right. Less than that and weeds push through and the moisture benefit disappears. More than four inches and you've built a sponge that stays wet, sheds water instead of absorbing it, and starves roots of oxygen. Beds that get topped off every single spring without anyone removing the old layer end up six or eight inches deep, which is worse than no mulch at all.
Now the volcano. Mulch piled in a cone against a tree trunk is the most common landscaping mistake in Pennsylvania, and it kills trees slowly enough that nobody connects the cause to the effect. Bark that stays permanently wet rots. Roots grow up into the mulch and circle the trunk, eventually girdling it. The fix is simple: pull mulch back so you can see the root flare — the place where the trunk widens as it enters the ground. If you can't see the flare, there's too much mulch.
Timing: mid to late spring, after the soil has warmed. Mulching over cold, wet ground in March traps the cold in. A light fall refresh is fine for appearance, but the real application belongs to spring.
And the part nobody mentions in the price comparison: the bed prep matters more than the mulch. Beds that are edged with a clean cut, weeded down to the root, and treated before the mulch goes on will look sharp in August. Mulch dumped over living weeds is a two-week improvement.