Ben Craig House is centrally located. The Scottish Tourist Board gives it a 4 star rating; the AA gives it a 4 diamond rating. Probably more important, it's got good reviews from people who've stayed there. An en-suite double-bedded room costs �30 to �35 per person per night, including breakfast.
It's one of several possibilities, with good reviews and cheap prices, on this website:
http://www.hostelworld.com
(Don't be put off by the word 'hostel' in that name. Simply use the 'Additional search options' to select 'Guesthouses' and 'Budget Hotels').
Chris