Angela,
Like so many things in our industry, and to be fair others, you will always, always get what you pay for.
Imagine:
You employ 2 cleaners on a site, they clean for 2 hours each, no problems with the cleaning from the client's side, everything going well.
Your client employs 30 staff on this site, they even have cctv installed, each staff member has their own key, your supervisor is also issued with the keys and relevant alarm codes, your supervisor losers his/hers keys.
What are you going to tell your client?
Does, not your client, expect you to cover this loss, quickly, comprehensively and with minimumn fuss, is that not why the client chose you in the first place? Because you offer a high quality service, coupled with reliability, and of course are professional in all areas.
If you choose to do without this cover, could you afford to replace the locks, perhaps not just on the main entrance, but the locked offices within the building as well, then of course the cost of supplying new keys, now! Today, not next week, or next month, when you have the cash?
It would be bad enough, to be insured correctly, and you would be looking to convince your client that stuff happens, after all human error does indeed happen, but I would guess that the client would not be very happy if they had to pay for this mistake, then wait until you re-imbursed them at a later date, at least with your company carrying the proper insurance, you and your client would be covered for this eventuality.
Your clients own insurance company, could very well insist on replacing all locks and issuing new keys, as a matter of course anyway, in order to maintain an acceptable level of security, and as a condition of continuing cover.
In my opinion, your insurance should give you peace of mind, knowing that you have covered ALL of the required risks that need to be covered, yes full cover is expensive, but part or no cover, whilst cheaper has the potential to destroy your business and your own personal reputation, this full and comprehensive cover could be used as a selling point for your company, if you point out to your prospects, the dangers and pitfalls of "cheap" cover.
There are various ways of obtaining cheaper insurance, not being truthful on the scope of work that you carry out, as an example, but with the powers that be, linking up databases, legislation etc, eventually these methods will be enforced.
The amounts paid out by insurance companies are getting very, very expensive, if you choose not to cover all reasonable risks, then if the turd hits the fan, you have only yourself to blame, regards,
Rob