I love a good bath! Since having my DD (4 months) I barely get chance now! It's the one little evening ritual I miss.
I always get my bath salts from TKMaxx or Homesense, they usually have Australian imported ones, big bags for about £7.99 but last ages, you only need a small hand full in your bath.
Home bargains do Westlab salts for about £1.99 which are similar.
What about some scented candles, and a bath caddy to read a book on - you can get wooden ones from Amazon.
For ultimate luxury Jo Malone do a Bath oil 30ml for £20, English Pear & Freesia is amazing, you only need 2/3 drops per bath so would last you a while especially if you only used it once a week or so.
Rituals do amazing bubble bath, one scented called Ritual of Sakura it smells sooooo good, £15 for 500ml.
Also get yourself a bath pillow!!