In 'The Royal Chambers' you can purchase infinite upgrades. After defeating the first wave of enemies, get the Sarcaphogus nearby by performing the following steps: Go to the side of the room that has a vertical water spout. Freeze it with Flow and climb up. You'll notice two pressure switches on either side of you. Get enough height and jump to one and then jump off the wall back to the water column. Jump to the other and hit that one too. Return to the column and now jump into the newly opened door to find the Sarcophagus.
You will probably get enough XP for one upgrade. Press 'TAB' and get any upgrade and then save your game. Now, find the exit and you'll come to another small room. This one has two water spouts and a waterfall. Freeze them with Flow and after you jump off the first spout, unfreeze everything so you go through the waterfall and can then land on the second spout once you freeze it all again. Now you will face Ratash. Do not run away. Instead, deliberately die. You will respawn back into the room. Now look in the top-right corner. You still have one more upgrade. Go buy something and save again -- you can repeat this as many times as you like.