a good test is too stick your car in 4th give it abit of revs and try to drive off. If your clutch is still good it should stall. If it doesn't and your still reving then chances are your clutch is slipping.
clutch change starts from around 250-300 from my experience + new clutch. gearbox is much more expensive, and it doesnt sound like you have gearbox troubles just clutch. I could be wrong..ha