Hi Harza,
The cost of Income Protection cover is determined by quite a number of variables. You've covered age, but the other ones would be gender, occupation, smoker (y/n), waiting period before benefit kicks in ("deferred period"), ceasing age and escalation rates in benefits pre or post claim. Without all of these details, it would be virtually impossible to guesstimate a costing. The other one is obviously "how much cover", but you may not be willing to disclose this on a public forum as it's related to your income.
However, for what it's worth, the cost of cover will tend to be linear, i.e. cover for 80% of your salary will be around 1.45 times the cost of 55%.