line that best fits nonlinear data

A mutual fund company has the following fee schedule:

On 50 miillion they charge: 0.75%
On 100 million they charge: 0.75%
On 200 million they charge: 0.70%
On 300 million they charge: 0.683%
On 400 million they charge: 0.675%
On 500 million they charge: 0.67%

I am trying to come up with a line that best fits, in MS Excel 2003. I can't do a linear regression since the data is nonlinear nor can I use the growth or linest functions in excel because they are for exponential growth.

1.) What functions in excel can I use to come up with a nonlinear equation that best fits the data so that I can forecast charges for money on say 750 million dollars???
2.) Also, can I use this equation to forecast charges on money below 50 million dollars? As a matter of fact, this is the most important part.

Thank you in advance for any help provided.

Any feedback would be greatly appreciated.


TS Contributor
On the chart within Excel, you can add a trend line, and it gives you options such as linear, polynomial (up to 6th degree), power, logarithmic, etc. I've had some pretty good luck with it.