388 is considered lucky because 3 rhymes with 生 ("live" or "lively") and 8 rhymes with 發 ("rich", "wealthy"). The whole phrase should be 生生發發, but of course, that couple would be going too far if the price was 3388K instead of 388K.
If the price was 150(K), the nearest lucky numbers are 138 "一生(都)發(達)" ("rich throughout your life") and 168 "一路發" ("wealthy all along"). But for 350, I think 388 is already the nearest lucky number. [Edit: 338 is actually nearer to 350 than 388 is, but I wonder if the couple is willing to cut the price.]
By the way, the numeral 4 rhymes with 死 ("death"). Some buildings in China don't have a fourth floor (i.e. 4/F is re-numbered as 5/F). 14/F, 24/F, 34/F, 4x/F etc. may be skipped too.