Is it possible to invest monthly through Vangaurd? What are the fees for this?

The only fees would be the custodial IRA fee($10 for funds with less then $5,000 or less than $50,000 with Vanguard) and the account maintainence fee of $10/year for accounts with less than $10,000 on Vanguard's index fund.

Sharebuilder cost $24 for the year plus the 0.1845% cost of running the Spider.

Wrong. The SPDR has an expense ratio of just .12% now:
http://www.amex.com/asp/indexshares.asp?symbol=SPY

Of course, you might go cheaper and take IVV with only .0945%:
http://www.amex.com/asp/indexshares.asp?symbol=IVV

The rate of return should be the same as the S&P500 just the fees will be diferent between the 2 companies?

'Should be' is the key word. Here's a link to an article illustrating the difference Vanguard has made through its quasi-active approach to tracking an index(Note the pre-tax returns since this is in a Roth IRA and the difference over 3 and 5 years):

http://news.morningstar.com/news/Wire/0,1230,3236,00.html

I'd also add that Vanguard 500 Index has for the past 3 years beaten the S & P 500 after expenses(Note: This isn't taking that $10 fee into account). Also, Vanguard has its own ETF shares in progress called VIPERs. Also, with the ETFs there is the possibility of buying in at a premium/discount since they don't always trade at a NAV like Vanguard's open-end fund does.

HTH,

JB

PS: My Roth holds 2 funds: Vanguard's LifeStrategy Growth and Capitol Opportunity and in a taxable account I hold Vanguard 500 and Growth index funds, just for disclosure.