The Sheriff will have granted the guardianship order on condition you provide some form of security, in this case a bond of caution. This is an insurance bond to protect against any negligence in administering your mother's finances.
OPG seem to have an agreement with only two companies to provide security for guardians - Royal and Sun Alliance and Zurich GSG. The premiums charged may be different if the bond is issued through a solicitor rather than an individual.
You may be able to go back to court to have the value of the estate - and caution - reduced. Should be straightforward for a solicitor.
Why not contact the insurers and ask for their premiums? That should help you decide whether going back to court is worth the effort and expense.
OPG produce a useful leaflet on caution
http://www.publicguardian-scotland.gov.uk/docs/caution%20leaflet.doc