I'm working for [omitted] software It is a very small comapny. We had developed an Intranet application which consists lot of complex calculation parts. We are looking for a Tool which helps in Testing most complex parts of our application i.e, It should have features of script developing/modifying for regression testing and it should also support Database Testing.
One more thing I would like to inform you that as ours is a very small company we are looking for a tool which comes under $500.00. If any OpenSource tools availble than it is well and good.
Your budget will exclude the larger commercial tools - particularly with ongoing licensing costs. I note that your company provides IT services, and you should consider whether any tool - and the skills acquired to support it - would provide ROI across the company, rather than on a single project.
An organisation's greatest investment in testing with any tool - licensed or open-source - is likely to be the cost of understanding the tool and developing+maintaining the tests. Open source is attractive not only because of its lower initial cost, but because the tools can be modified to suit your purposes. I've detailed some open-source tools below.
Although your email is more detailed than many, it doesn't address the most important question - the purpose of your testing. If you would like to discuss this, call me using skype.
For functional testing of your underlying complex calculations, your best bet might be to write a simple test harness to supply input and check output, avoiding the user interface. This approach allows you to automate testing using powerful design techniques that can go far beyond simple regression scripts, and would potentially give you rather more confidence in the face of change. Look into ideas around combinatorial testing, fuzz testing, and model-based testing.
For testing a web interface, and some functional testing of the software accessed through a browser, I would suggest watir. If by database testing, you mean testing for the purpose of assessing performance or scalability of a database, I would suggest looking at jmeter or grinder.
Watir : http://wtr.rubyforge.org/