You can rent a bungalow on the beach from around $30 a night, up to maybe $100 depending on where you stay. You could also travel away from the main beach to find a quiet corner of the island for yourself and for much less. I once stayed somewhere remote which was $18 a night but the generator was off until about 6pm everyday!
Then of course there's Nha Trang. Nha Trang is do-able in a weekend from HCMC. Most city dwellers choose the nearer Mui Ne as a weekend getaway, but how about this - take the overnight train on Friday night to Nha Trang. Journey time is about 9/10 hours depending on which service you take, so leaving around 9 or 10pm will you get you in about 6 or 7am. Get a soft sleeper bed from about $7-$12 depending on top, middle or bottom bunk, the bottom one being the most expensive. Compartments are shared with 6 people. This gives you two very full days to relax, and you return on the overnight train on sunday evening, arriving in time to nip home for a shower and some breakfast. OK, maybe your a little tired on Monday, but it's worth it for two days of this, isn't it?