LPPYOga, You would use following shortcode for pay now button with amount of $40.
[paypal_button type="paynow" amount="40" name="For Two People"]
[paypal_button type="paynow" amount="50" name="For Three People"]
[paypal_button type="paynow" amount="60" name="For Four People"]
You can have as many buttons as you want.
If you want a single button, where you can visitor can choose number of persons and charges are adjusted as per selected number of persons, It is not supported yet. But you can use workarounds to do that. To get you started
1. Wrap your button shortocode with like
<span class="paypal_button">[paypal_button ...]</span>
2. Use jQuery to apply calculations on click
var qty = jQuery(this).siblings('input[name="quantity"]').val();
// $20 per person
// $20 for first two and the $10 for each remaining person
var amount = ((qty-2)*10)+40;
Disclaimer: above code is not tested. You may expect typos and errors.
About the second question, it is not possible with the form buttons, But you can use "echo_link=1" attribute to create a link that can be used to open a new window.
You can check documentation page
Hope it solves your problem.