There is not really a built in way to preselect the most expensive modifier by default.
Having said that, then you should be able to get around that as long as you setup some publishing standards. The easiest way should be to make sure that either the first or last modifier is the most expensive.
If you have done that you should simply need to add the modifier as a hidden field. It would look something along the lines of the following:
{exp:store:product entry_id="{entry_id}"}
<h2>{title}</h2>
{if modifiers}
{modifiers}
Price: {price_inc_mod}
{modifier_options}
{if option_last}
{!-- this could also be if option_first if you decided to make the first option most expesnsive --}
<input type="hidden" name="{modifier_input_name}" value="{option_id}">
{/if}
{/modifier_options}
{/modifiers}
{else}
{!-- if no modifiers show regualr price --}
Price: {price}
{/if}
<input type="text" name="item_qty" value="" />
<input type="submit" name="submit" value="Add to Cart" />
{/exp:store:product}
This assumes that you are passing in the entry id and that each product has its own form and quantity. IF you were wanting all the results to have a single form submit with the ability to add multiple items to the cart at a single time you would need to wrap this in the Product Form tag pair and adjust the template at bit.