Two well-regarded Rob Shearman audios, The Holy Terror and The Chimes of Midnight, are available for £5 (about $8 in the US) on CD or £2.99 ($2.99 in the US) for download for a little while longer. Link.
It should be noted that Big Finish has made this offer because Steven Moffat recently mentioned in an interview these two stories were his favorite Sixth and Eight Doctor stories along side the actual series.
Another Big Finish offer: for two weeks the Eighth Doctor Adventures, a series of single disc adventures starring McGann with Sheridan Smith as companion Lucie Miller, are available for £5/$8.13 apiece for CD or £5/$5 for download. You can also subscribe to each series for a slightly better deal. Details here.