Mussels or shellfish are really expensive these days. If you order mussels at any zi-char stalls will definitely cut you back around $12 for a small portion.
I found this gem at Golden Mile… 1 BIG PACKET of very very fresh mussels for only $5!!! Simply very fresh mussels cleaned and blanched in alot of asian herbs such as kaffir leaves, basil, chilli, ginger, lemon grass, onions and salt. Done-ness just nice – very fresh and succulent!
And it comes with a small container of spicy thai chill sauce (not the sweet ones). Simply eat it on its own! You may purchase this from takeout counter and ask to eat in-restaurant. Been a fan for years… I just whack 1 pack all by myself ytd… 
This is at Diandin Leluk Thai Restaurant, there is this little section where they sell takeaways such as desserts, savouries, etc. Another good buy is their mango sticky rice but thats a tad expensive at $6. Diandin Leluk is a (used to open 24hr) thai restaurant. For now, am just recommending their super worth-it mussels! Limited stock daily…
Mussels shiokness!!!
Diandin Leluk Thai Restaurant
5001 Beach Rd
#01-67/68/69
Golden Mile Complex
Singapore 199588
Opens daily
11am – 10pm