I believe it generally depends on the size of your SIPP "pot of cash" - for instance I believe once your pot gets above £100k Hargreaves Landsdown definitely isn't the one you want to be using!
The below link is a recent article for ISA charges - however I think you can draw a pretty strong parallel between ISA charges and SIPP charges...
http://www.telegraph.co.uk/finance/...pest-fund-supermarkets-for-Isa-investing.html