What is the benefit of implementing an ERP software by knowing the fact that its very expensive?

The true benefit of any ERP is the speed, accuracy and timeliness of information that can be used as a decision making tool. For instance, the trends in sales of a particular product, season or class of customer can give a really meaningful data to work on Customer Profitability Analysis. Similarly, various cost and expense reports can be used as a basis to re-design a product, service, process or design such that it eliminates or reduce costs without diminishing the associated value. This is often termed as Value Engineering and is a vital pro of using a good ERP.

However, If an ERP is just seen and used as an Accounting (or transaction processing software), then it may not be worthwhile to go for it. 

Not expensive only the implementation, but also, the training the team. However, the software comes many advantages when the proccesses are integrated. The time optimized is the key of productivity. I had already worked SAP and I agree with the advantages of this product.