If you know how to tune it yourself, it makes little sense to take it to a shop. If you have no idea how to keep it running well and it is giving you trouble, then a shop or a knowledgeable friend are your only options. The price varies totally depending on what is done. If it's just to tweak the derailleurs and the brakes, then the cost will be lower than if you're talking about truing wheels, adjusting hubs, headset, BB, etc. The range of what constitutes a tune-up varies widely.