Does your car pull to one side or wander over the road? Mine did with stuffed caster bushes.
With the bushes you can replace them with standard ones or go for adjustable ones. Most people go with adjustables, as this means you can increase the caster giving you more straight line stability and more camber while cornering. Whiteline, Superpro and Pedders all make adjustable caster bushes.
I paid $130 for adjustable bushes and $40 to get them press fitted, but I'm assuming you wont be doing the work yourself, so it'll be a little bit more than that when you factor in labour costs for rermoving the tension rods. Ring around and get some quotes
Btw, coilovers wont fix the problem at all - different part.