Expensive is relative and should be considered against the financial risk not meeting scalability requirements. For some environments even the most expensive tool is cheap compared to the risk of the application not scaling and the associated financial risk of this event: Think of your website failing on the largest shopping day of the year and your competitors getting your cast off business.
Depending upon the technologies involved in your performance testing you may have many tool options available from open source through commercial. But whatever tool route you go be sure it meets the following
- It can exercise your interfaces just as a user would
- The reporting for the tool matches what you need to report against your requirements and is conducive to finding bottlenecks in your environment
- Your staff has the skills to use the tool successfully
- Your management is willing to invest in training and a mentor for your first 2-3 engagements
Get a mismatch on any of the above and you may as well have purchased the most expensive performance testing tool on the market and hired the most expensive consulting firm to deploy it even if you are using a "free (as in beer)" performance test tool.