Let's say I held £100,000 in Interactive Investor and wish to add £20,000 to my investment each year in monthly instalments. They charge £10/month for a standard account that offers one free trade per month with no percentage based platform fee so on the surface it appears I pay £120 in fees a year.
Existing Investment: £100,000 Annual Platform Fees: 12 * 10 = £120
However there's also a mark-up on the exchange rate when investing in non GBP funds of 1.5%. I invest entirely in US based index funds so I would pay 1.5% on the £20,000 invested and then pay another 1.5% when it is eventually taken out. There's a big assumption the fee stays the same but I can't see any other way of calculating this.
Annual Investment: £20,000 Annual Exchange mark-up Fees: 20,000 * 0.015 * 2 = £600
Do mark-up exchange rates really have this much of an impact?
If I have calculated this correctly why do many reputable sites brokers omit their exchange fee margin from their fee schedule  while advertising their less relevant platform fee? Popular comparison sites also fail to mention this cost.
If I have calculated incorrectly or based it on faulty information where have I gone wrong?