On average, listings lease in the $10's per square foot per year. The Galveston region is going through a decline in rates. In the last 3 months, the median rate decreased $4 per sqft per year. That's a 18% decline in rent.
Compared to the rest of the Houston Metro market, the Galveston region has lower rates, so you should have no trouble finding a great deal in this depreciated region.
In the last 90 days, 29 listings have been posted online, which means listings most likley are leasing quickly. You'll want to be prepared to make an offer immediately. There aren't many listings below 2,000 square feet posted, so finding a space for a small business might be tough.
The standard measurement for office space in this area is around 1,300 sqft. Most buildings in this region are around 2 stories. Most buildings in this region were built in 1973 and are Class B.