You can configure per diem and approval processes based on amount on ByDesign.
On business configuration there is an activity "Expense Reporting - US" or "Expense Reporting - Canada" where you can define process groups for different levels of approvals for certain groups of employees, or if you want to set limits on the amounts that line managers at the different management levels are authorized to approve.
If you maintain "Expense Types" you can specify default amounts and maximum amounts for expense types, including which conditions apply if maximum amounts are exceeded. The amount you enter applies per attendee or recipient. You can even include different per-diems per region for US configuration.
Also there is a fine-tuning activity called "Business Task Management for Expense and Reimbursement Management Activity" where you can edit audit and approval scenarios, for instance, to require approval only if a minimum amount is exceeded.