Costs are a factor. Even my cousin, another amateur genealogist who actually lives in Scotland, makes due with the extracts because of cost. 
You should never, ever, trust anything you find online unless it is an image of an original document. Transcriptions and indexes and (especially) other people's trees are very useful as pointers to where to find the original information, but no substitute for checking the original.
G F Black's
The Surnames of Scotland doesn't mention Lauchwarrit as a synonym for Lockhart. However he says, "Janet Laucharrit recorded in Edrom in 1670 most probably derived her name from Lochquarret (now Vogrie) in Midlothian".
See
http://www.geograph.org.uk/gridref/NT3763 and
http://www.geograph.org.uk/gridref/NT3760Black's reference is to the Lauder Commissariot records
Laucharrit, Janet, 11 July1670 in Edrem, spouse to William Currie, testament dative & inventory, Lauder Commissary Court CC15/5/6
This is one orginal that is probably not worth getting. A 'testament dative' means that she left no will, so there is not likely to be useful information about her family connections.
BTW don't ask for a photograph of anything from the SP centre. In the search rooms, you are not allowed to take photographs of the screen.
