Mandarin Hotel at Silom is about 1020 Baht via internet. On booking, donīt forget to book for 2 persons and put the 'nick name' of your gf on your voucher, so you can present it on checkin.
If you checkin alone, ask at checkin for breakfast vouchers for 2 !!!!
Last time I checked in and picked up my gf about 3 hours later on sathon. No problem.
http://www.latestays.com/mandarin