Hi Doruk. I' afraid I do not know what 870 MW CCPP is . . however, the question is about estimating a budget.
Usually (??) in the UK the safety dept is just about providing people and expertise. Therefore staff costs are the main element of the budget. Which will of course depend on how many and how much they are paid, plus 'on-costs'.
The premises, furniture and associated running costs will depend on the organisation - you may have little say on the charges made against the budget, although the budget will have to contain the funds to pay. IT hardware and software (licences), support, phones, print/copying, a/con etc. may be included or added to premises category.
More variable costs are: travel & subsistence (getting about etc.), staff training costs, medicals, again dependant on how many staff.
Then finally there is the 'choice-related' funding - resources. Apart from stationery, this could include staff PPE, journal subscriptions, specialist software or information systems, training equipment, materials or resources, cameras, measuring kit (as required). This will require a larger one-off expediture for the initial set up then an annual amount (don't forget calibrations, upgrades, renewals every 3 years).
Some safety 'department' budgets may also include amounts for the organisational expenditure on things like PPE (for all), signs, training buy-ins, fire extinguishers and other types of hardware with a H&S association. I have never had such a set-up, better that operational managers are responsible for purchasing what they need to do a job safely from their own budget.