Like Brett said: drill it and get a sump.
With a light bio-load and aggressive skimming you could get by with a 10-20% water change per month. So, say aprox. 40 gals per month. Bucket of salt makes 160 gals of SW and cost about $50. Monthly cost of salt = $12 The big monthly expenses are electricity and additives/supplements such as calcium, alk, etc.
P.S. Very few people are able to keep a light bio-load. Nearly everybody overstocks their tanks. It's hard to resist one more fish or coral or clam.
