Most recently, we stayed in family accomodation - Mikki's sister lives down that way, and her sister in law has a chalet on one of the sites.
We'll be going down in March again ourselves. Probably use one of the holiday sites - John Fowler is cheap. Cottages tend to be quite pricey (200 quid plus per week), even at that time of year.
No need to actually stay in St Just - the whole of Penwith is small enough that you can get anywhere within 20-30 minutes - we tend to stay up near St Ives as it gives a quick getaway for the sites outside of Penwith itself - give us a call later tonight and we can chat about it...