How much does an ILS approach cost?
For New Zealand1, the Airways Pricing Framework is a document which describes how the prices are set but not the actual prices themselves (that is in a different document that it appears you have to sign up to receive). Some excerpts:

Consistent with international practice and ICAO guidelines, Airways uses aircraft weight as the basis for its charges.
En-route charges also vary with distance flown. Intrinsic to the formulae is the principle that prices should be
transparent and practicable. A simple pricing system lowers Airways’ costs and those of its customers.

For IFR approaches:

For aircraft flying IFR, the greater of the Minimum Price or:

for aircraft weights under 5 tonnes = Base Rate x MCTOW / 5
for aircraft weights 5-30 tonnes = Base Rate + Weight Rate x (MCTOW – 5)
for aircraft weights above 30 tonnes = Base Rate + Weight Rate x 5 x Square root of (MCTOW – 5)

» where the Minimum Price, Base Rate and Weight Rate are provided by the relevant price table.
Price tables provide the prices for each service and location. Price tables are provided in Airways’
standard terms and conditions
» where MCTOW is an aircraft’s maximum certified take-off weight measured in tonnes.

There are also different formulas for aerodrome services, en-route domestic, and en-route oceanic service.
More information can be found at the New Zealand Service and Pricing Framework page.
1. Yes I know that New Zealand is not a country in Europe.