mariegold, I note your question is unanswered. My 2014-2015 solicitor's fees for purchase of 7.5 acres came to £880 for the conveyance and just over £270 for searches. To note, I didn't bother with offered Environmental, Mining and Water searches as I had a pretty good idea what they would turn up (or not turn up). I was tempted to also forego the Local Authority search (over £200 as I recall) having gained a fair knowledge of local plans, but decided not to risk it in the end: as fully expected though, the Local search came up with nothing note-worthy.
As regards searches (when on a tight budget), it comes down to a judgement based on your knowledge of the area and any personal research: for instance, I was also offered a China Clay search (being in China Clay country) for £90 as I recall, but I was able to establish via the web that there are no surveyed clay deposits under my land. Your conveyancing solicitor will probably also offer every "search" there is! (They have to to protect themselves.)