Hi Bejeweled,
You can do one of the following:
- Query your SharePoint list with a filter (qRules FilterOwssvr, SharePoint 2010 REST, or DBXL QuerySharePoint are three ways to do this)
This technique is best practice for large lists since the filter is done on the server. - Get all items from SharePoint list and filter in the form
Simple and effective if your approver's list is less than a few hundred people.
So, I'll assume you can do #2 for starters.
Your approver's dropdown should be tied to the SharePoint List data connection. You don't need to use a button for #2 (that's more of a #1 option).
As your form is filled out, the total cost is changing. I'm assuming you have a field for total cost in the main data source with a default value equal to the sum of the line items.
In the dropdown for the approvers, just click on filter and set up two conditions in that filter dialog. One says that the Approval Minimum is less than the total cost field *and* the other says the Approval Maximum is greater than the total cost field.
That should be all that's required. If you have more than a few hundred approvers, check out the qRules documentation for FilterOwssvr. I know you have a copy of qRules, and that should be faster than REST. DBXL QuerySharePoint is fastest for really large lists (more than 1000 rows)
Go forth and conquer!