On the Importance of Good Eyepieces…
Every photon of light collected by a telescope passes through an eyepiece before it reaches your eye. That light is therefore afflicted by every optical problem in that eyepiece - and the cheaper the eyepiece, the more problems it has. A $10 eyepiece will make a $2000 telescope perform like a $10 toy. If your telescope's eyepieces are labeled H (Huygenian), R (Ramsden), or SR (symmetrical Ramsden), they are basically toy eyepieces and should be replaced.
Since eyepieces are half of the optical system of your scope, and usually the most complex half, they should be at least the same quality as your scope (preferably better). A good telescope won't improve a poor eyepiece - but a good eyepiece will get the most out of a so-so telescope.
When buying eyepieces, however, resist the urge to immediately get the highest power eyepieces you can find. They require an experienced observer and excellent seeing conditions (dark, clear, steady skies) to get the most from them. Medium power eyepieces, on the other hand, are usable under a much wider range of seeing conditions.
On the Importance of Good Seeing…
Seeing conditions - the steadiness and transparency of our atmosphere - determine
how much power you can use each night. The seeing conditions also determine just how much detail is visible at any given power, particularly with larger scopes -
more so than the type of eyepiece, or how much that eyepiece costs.
If the image in your telescope is unsteady, fuzzy, or unsharp at high power, you may be trying to use more power than the seeing conditions will allow. Changing to a lower power eyepiece will usually improve the image more than buying a better-quality high power eyepiece. In other words, a more expensive high power eyepiece won't improve poor seeing conditions - but a lower power eyepiece will make so-so seeing more enjoyable.
Consistently poor performance at medium and low powers, however, may mean that you need better eyepieces, or that your scope is out of collimation.