The following tables set forth the costs, accumulated amortization and net book value of the Company’s separately identifiable intangible assets as of March 31, 2021 and September 30, 2020 and estimated future amortization expense.

Estimated Amortization Expense

The trade names are amortized on a straight – line basis over their respective estimated useful lives of between five and ten years. Intangible assets that represent customer relationships are amortized on the basis of estimated future undiscounted cash flows or using the straight – line basis over estimated remaining useful lives of five to ten years. Non-compete agreements are amortized based on a straight-line basis over the term of the respective non-compete agreements, which are typically five years in duration.

The amortization expense for intangible assets was $1,015 and $2,059, and $1,398 and $2,795 for three and six-month periods ended March 31, 2021 and 2020, respectively.