You would need to create a subscribe button that allows for a user input of the amount:
Example below:
Code:<form target="paypal" action="https://www.paypal.com/cgi-bin/webscr" method="post">
<input type="hidden" name="cmd" value="_xclick-subscriptions">
<input type="hidden" name="business" value="youremailaddress@yourdomain.com">
<input type="hidden" name="lc" value="US">
<input type="hidden" name="item_name" value="Monthly Donation">
<input type="hidden" name="item_number" value="MD-2010">
<input type="hidden" name="no_note" value="1">
<input type="hidden" name="no_shipping" value="1">
<input type="hidden" name="return" value="https://www.yourwebsite.com/success.html">
<input type="hidden" name="cancel_return" value="https://www.yourwebsite.com/cancel.html">
<table>
<tr>
Enter your recurring amount:
<td>
<input type="text" name="a3" value="">
</td>
</tr>
</table>
<input type="hidden" name="currency_code" value="USD">
<input type="hidden" name="src" value="1">
<input type="hidden" name="p3" value="1">
<input type="hidden" name="t3" value="M">
<input type="hidden" name="srt" value="12">
<input type="hidden" name="sra" value="1">
<input type="hidden" name="bn" value="PP-SubscriptionsBF:btn_subscribeCC_LG.gif:NonHosted">
<input type="image" src="https://www.paypal.com/en_US/i/btn/btn_subscribeCC_LG.gif" border="0" name="submit" alt="PayPal - The safer, easier way to pay online!">
<img alt="" border="0" src="https://www.paypal.com/en_US/i/scr/pixel.gif" width="1" height="1">
</form>
You would need to change some of that info to reflect the correct stuff for what you want to do but it should work for ya.