You don't need the AND
option in your query, as it is by default, and your query is only composed with 'ands'
$conditions = array(
'Product.cost >= ' => $startPrice,
'Product.cost <= ' => $endPrice,
'Product.id > ' => $lastid,
'Product.category' => $category
);
EDIT
If you need an OR
statement in category
then you can just add:
$conditions = array(
'Product.cost >= ' => $startPrice,
'Product.cost <= ' => $endPrice,
'Product.id > ' => $lastid,
'OR' => array(
'Product.category' => $category,
'Product.category' => anoter_value
)
);