What is the difference between Std. Buy Price, Std. Accounting Cost, and Supplier Price?

All of the Purchasing Pricing Fields are used to auto-populate a Purchase Order with a specific default for the particular Product ID and Supplier you add to it.

If you have Product ID: ABC  and ABC has a Std. Buy Price: $1.00  and a Supplier 1: Supplier XYZ and Supplier 1 Price: $2.00 then on a NEW Purchase Order:
IF the Supplier of the new PO is Supplier XYZ then the default price for Item ABC will be $2.00 on the PO.
IF the Supplier of the new PO is left blank or is Supplier QRS, then the default price for Item ABC will be $1.00 on the PO.

Supplier X Price is a default price field for the PO when that product id is added to a PO and that Supplier is chosen as the Supplier the PO is issued to.
These "Price" fields are all default fields for criteria chosen on the PO. 

So Std Buy Price is a default field the PO references to get a price for an item that does not have a specified Supplier listed or matched on the PO.
Std. Buy Price is used to "Default" the PO unit price when a product id is added to a PO.  It is not the cost field.

Finale has two "cost" Fields or values.  Std. Accounting Cost and Average Cost.
Average Cost - Average cost is a system calculated field that is updated when a product id is placed on a Purchase Order at a specified unit price and is received into the inventory on a shipment against the purchase order.
The Average Cost field is not editable, however, you can create an Average cost change journal entry transaction to establish, change, or update the system average cost. Go here to learn more about making changes to average cost.

Std. Accounting Cost - This is a static field that you have to update and maintain a value for.  It is not historically maintained, meaning if you change the value of the field today, it affects all records and transactions for any time period when reporting. If that field had a value in it 6 months ago and you run a stock history report for stock value as of 6 months ago, that stock value report will have today's value for std. accounting cost as its basis.  Average Cost is historical.

If you need to regularly make adjustments to the Std Accounting Cost field on multiple product ids, simply keep an excel sheet with two columns, A and B, with Product ID in one and Std Accounting Cost in the other.  Use it to bulk copy/paste import/update the fields.

Finale has numerous Stock Valuation reports that you can use to see your stock on hand valued at Avg Cost or Std Accounting Cost.
Finale Inventory does not currently have any support for LIFO or FIFO.