I've looked and this is what I've found. The syn values are per tbsp.
Tartare sauce - 2.5 syns
Mint sauce - 0.5 syn (if you use Garden mint as opposed to mint sauce, the garden mint is free)
Dijon mustard - 0.5 syn
Wholegrain mustard - 0.5 syn
Brown sauce - 1 syn
Bechamel sauce - for this I can only find an Asda Chosen By You Bechamel sauce which is 22 syns for a 350g jar.
Horseradish sauce - 1 syn (if luxury, it's 2 syns)
Tabasco sauce - pepper, hot habanero and smoked chipotle pepper are all free, the other varieties are 1 syn
Garliuc sauce - 2.5 syns
Plum sauce - 1.5 syns
BBQ sauce - 1 syn
Salad cream - 2.5 syns
Golden syrup - 2.5 syns
Treacle - 2 syns
Rosewater - no results
Orange water - no results
Almond extract - most are free, but can depend on brand names, etc.
Vanilla extract - most are free, but can depend on brand names, etc.
Peppermint extract - most are free, but can depend on brand names, etc.
Sun dried tomatoes - NOT in oil - I can only see them in oil, and drained they would be 6.5 syns per 100g