I also worked with PLA2 from snake venom (Welcome to PLA2's world). We worked with the method described by De Haas and Postema (1968). Basically,
the Phospholipase activity is measured using egg yolk as a substrate in the presence of 0.03 M sodium deoxycholate and 0.6 M CaCl2. And we monitor the activity using a pH meter and when the pH value decrease 0.3 unites we add 3ul of a titrated solution of NaOH. The activity is expressed in mEqNaOH/mg/min. This is one option of cheapest and with high accuracy method for PLA2 activity.
If you have any question, do not hesitate to ask me.
My group has successfully used the Invitrogen The EnzChekTM Phospholipase A2 Assay Kit. It may not be the cheapest, but it is easy and the reproducibility of the results is very good.