£10m seems very low anyway
500,000 pensions administered for £10m? Even if the term is only 5 years that's still only £4 per pension per year.
No doubt there will be a UK onshore requiements and possibly security clearances needed. Just the 3rd party costs will be more than this: hosting environment, software licences, identity and access management, annual audit. Then add the providers costs (assumed leveraged) for call centre people, telephony, NOC, SOC, servers etc etc.