I have pretty extensive knowledge of fillers - I was involved in early assessment of the first two hyaluronic acid fillers developed - Restylane and Juvederm, and continue to follow the market. Amazing how many uses for HA have been found since then. Anyway, several substances are available as fillers for penile enhancement, but Juvederm VOLUMA is arguably the best.
First, $350 is dirt cheap. Is this doc in a third world country and using off brand fillers? Otherwise, I'd expect it to cost ALOT more. In the US, the cost is probably $3K+ for a full procedure done by a good doc, all included.
Second, while Juvederm is pretty safe and effective for PGE (penile girth enhancement), it usually doesn't do much for length and when it does, it's often just flaccid length on guys who start out with below average length.
A few things to consider: Outcomes are HIGHLY dependent on the skill of the doc. Poor administration can result in several issues ranging from bothersome/ugly lumpiness to serious adverse events like a ruptured blood vessel. Satisfaction ratings are pretty good, but dissatisfaction does happen and it's often cited by the sexual partner of the patient, often due to erections being softer. The natural part of your dick still gets rock hard, but the HA in there stays soft. Depending on how much they inject and how good the doc rolls it in, you can have up to a month of no sex down time, but this varies.
The effect when using Juvederm VOLUMA can be up to 18 months before it starts to reabsorb. Some people reabsorb it faster, so you never know. If the doc botches the job and it's misshapen, there are substances that can be safely used to dissolve the HA and undo the effect.
I could write a few pages on this, but will stop here. Basically, if you want girth and don't mind not being as hard as usual, it's not a bad choice. If you're going for length, good chance you'll be disappointed.