worth noting, if you're a married couple the homemaker may* qualify as a dependent of the main earner. The difference between the full contributory pension and the amount for a qualified dependent is only €25, so 1 years voluntary PRSI of €500 would take you 20 years to recoup...
also if you don't have the average 48 weeks contributions, the next band down is only €5 per week less, & it covers everyone with average 20 weeks contributions or more, so your voluntary €500 would take 100 years to recoup