I would first save the money. If I had to spend it, I would do the VO2max test.
Lactate threshold can be very closely/accurately approximated with a field test. Also, it can change a lot, even over the course of a month - meaning you need to retest often. Lastly, it is a pretty inaccurate measurement. You will get pricked and have blood drawn every 1-2 min or so, and the person in charge will try to catch you as close to 4mmol/L as possible without going over. If you do go over, that person has to estimate your threshold - possibly extremely inaccurate.
VO2max doesn't change much, and is mostly genetic. To find out what your max potential is as a cyclist, I would have this done. It's a pretty useless number other than bragging rights, or to make a correct career choice - but at least it isn't worthless in a month.