Which element to emphasis in a list

On the site I'm working on, we have a list of buttons which make donations.

We would like to promote a specific donation button so it will convey more traffic. We tends to prefer less big donations over a lot of small ones (at least it's the analysis we made until now).

Example, emphasis on the 2nd element:

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Some though we have on the subject:

• If we promote the first link, it will make people click the first link, thus driving less "big" donations.
• If we promote the last element, it will repel people.
• The second may be a good option, but it's a guess.

Also, the amount and number of buttons are variables and unknown from us, so we would like a generic way to do this.

Right now the emphasis is done by a different background color, but any other emphasis be OK.

Is there a proper way to choose which element to promote?